Integrate Twilio for Inbound Calls on Railway Deployments: A Step-by-Step Guide

TL;DR

Most Twilio integrations on Railway fail because webhooks timeout or lose state between container restarts. Here's what you build: a Node.js server that receives inbound calls via TwiML VoiceResponse, routes them through Railway's stateless architecture using Redis for session persistence, and handles call state without losing context on redeploy. Stack: Twilio SDK, Express, Redis, Railway. Result: calls that survive infrastructure changes.

Prerequisites

Twilio Account & Credentials
Active Twilio account with a purchased phone number. Grab your Account SID, Auth Token, and phone number from the Twilio Console. You'll need these for webhook authentication and API calls.

Railway Account & Deployment
Railway project with Node.js runtime (v18+). Your Railway app must be publicly accessible via HTTPS—Railway provides this by default. Note your deployment URL; you'll use it for Twilio webhook callbacks.

Node.js & Dependencies
Node.js 18+ installed locally. Install express (v4.18+) for HTTP routing and twilio (v3.80+) for TwiML VoiceResponse generation. Optional: dotenv for environment variable management.

Network & Security
HTTPS endpoint (Railway handles this). Twilio validates webhook requests using your Auth Token, so keep it in environment variables. No firewall restrictions—Twilio's IPs must reach your Railway deployment.

Local Testing
ngrok or similar tunneling tool to test webhooks locally before deploying to Railway. Twilio requires a public URL for inbound call forwarding.

Twilio: Get Twilio Voice API → Get Twilio

Step-by-Step Tutorial

Configuration & Setup

Deploy a Node.js server on Railway that exposes a public webhook endpoint. Twilio requires a publicly accessible URL to forward inbound call events—Railway handles this automatically via its domain provisioning.

Install dependencies:

npm install express twilio dotenv

Configure environment variables in Railway's dashboard:

• TWILIO_ACCOUNT_SID - Your Twilio account identifier
• TWILIO_AUTH_TOKEN - Authentication token for webhook validation
• PORT - Railway assigns this automatically

Critical: Twilio webhooks timeout after 15 seconds. Railway's cold start can take 3-5 seconds on free tier. Use a persistent connection or upgrade to prevent dropped calls.

Architecture & Flow

flowchart LR
    A[Caller] -->|Dials Number| B[Twilio]
    B -->|POST /webhook/voice| C[Railway Server]
    C -->|TwiML Response| B
    B -->|Executes TwiML| A

When a call hits your Twilio number, Twilio sends a POST request to your Railway-hosted webhook. Your server returns TwiML (XML instructions) telling Twilio how to handle the call—play audio, forward to another number, or start a recording.

Step-by-Step Implementation

1. Create the webhook handler

const express = require('express');
const twilio = require('twilio');
const app = express();

app.use(express.urlencoded({ extended: false })); // Twilio sends form data

app.post('/webhook/voice', (req, res) => {
  // Validate webhook signature to prevent spoofing
  const signature = req.headers['x-twilio-signature'];
  const url = `https://${req.headers.host}${req.url}`;

  const isValid = twilio.validateRequest(
    process.env.TWILIO_AUTH_TOKEN,
    signature,
    url,
    req.body
  );

  if (!isValid) {
    return res.status(403).send('Forbidden');
  }

  // Build TwiML response
  const twiml = new twilio.twiml.VoiceResponse();
  twiml.say({ voice: 'alice' }, 'Call received. Connecting you now.');
  twiml.dial('[phone]'); // Forward to your number

  res.type('text/xml');
  res.send(twiml.toString());
});

const PORT = process.env.PORT || 3000;
app.listen(PORT, () => console.log(`Server running on port ${PORT}`));

Why signature validation matters: Without it, attackers can spam your webhook with fake call data, triggering unwanted actions or racking up Twilio charges.

2. Deploy to Railway

Push your code to GitHub, connect the repo in Railway's dashboard, and deploy. Railway auto-generates a domain like your-app.up.railway.app.

3. Configure Twilio webhook

In Twilio Console → Phone Numbers → Active Numbers → Select your number:

• Set "A Call Comes In" webhook to: https://your-app.up.railway.app/webhook/voice
• Method: HTTP POST

Error Handling & Edge Cases

Webhook timeout (15s limit): If processing takes >15s, Twilio hangs up. For long operations (database lookups, API calls), return TwiML immediately with <Pause length="5"/> while processing async.

Cold start delays: Railway's free tier can sleep after inactivity. First call may timeout. Solution: Use Railway's persistent connection or ping your endpoint every 10 minutes with a cron job.

Invalid TwiML: Twilio silently fails if XML is malformed. Always validate with twiml.toString() and check Twilio's debugger logs.

Testing & Validation

Test locally with ngrok before deploying:

ngrok http 3000
# Use ngrok URL in Twilio webhook config

Check Railway logs for incoming requests. Twilio sends From, To, CallSid in the POST body—log these for debugging.

Common Issues & Fixes

"Webhook returned non-200 status": Your server crashed or returned an error. Check Railway logs for stack traces.

"No response from webhook": Railway domain not publicly accessible. Verify deployment status and domain provisioning.

Calls drop immediately: TwiML response missing or malformed. Return valid XML with at least one verb (<Say>, <Dial>, <Play>).

System Diagram

Call flow showing how Railway handles user input, webhook events, and responses.

sequenceDiagram
    participant Passenger
    participant TicketingSystem
    participant PaymentGateway
    participant TrainSchedule
    participant NotificationService
    participant ErrorHandler

    Passenger->>TicketingSystem: Request ticket booking
    TicketingSystem->>TrainSchedule: Fetch train details
    TrainSchedule->>TicketingSystem: Train details response
    TicketingSystem->>PaymentGateway: Initiate payment
    PaymentGateway->>TicketingSystem: Payment success
    TicketingSystem->>NotificationService: Send booking confirmation
    NotificationService->>Passenger: Booking confirmation message

    Note over TicketingSystem,PaymentGateway: Payment failure scenario
    PaymentGateway->>TicketingSystem: Payment failure
    TicketingSystem->>ErrorHandler: Log error and notify user
    ErrorHandler->>Passenger: Payment failed notification

    Note over TicketingSystem,TrainSchedule: Train not available scenario
    TrainSchedule->>TicketingSystem: Train not available
    TicketingSystem->>ErrorHandler: Log error and notify user
    ErrorHandler->>Passenger: Train not available notification

Testing & Validation

Most Twilio integrations fail in production because devs skip webhook signature validation. Here's how to test locally and catch auth failures before deployment.

Local Testing

Expose your Railway deployment via ngrok to test Twilio webhooks without pushing to production:

# Start ngrok tunnel to your Railway deployment
ngrok http https://your-app.railway.app

# Copy the ngrok HTTPS URL (e.g., https://abc123.ngrok.io)
# Update Twilio webhook URL to: https://abc123.ngrok.io/voice

Test inbound call flow with a real phone call. Watch Railway logs for incoming webhook requests. If you see 403 Forbidden, signature validation is failing—check your TWILIO_AUTH_TOKEN environment variable matches your Twilio console.

Webhook Validation

Verify Twilio's request signature to prevent spoofed webhooks. This catches 90% of security issues:

// Validate webhook signature before processing
const crypto = require('crypto');

app.post('/voice', (req, res) => {
  const signature = req.headers['x-twilio-signature'];
  const url = `https://${req.headers.host}${req.url}`;

  const isValid = twilio.validateRequest(
    process.env.TWILIO_AUTH_TOKEN,
    signature,
    url,
    req.body
  );

  if (!isValid) {
    console.error('Invalid signature:', { signature, url });
    return res.status(403).send('Forbidden');
  }

  // Process valid webhook
  const twiml = new twilio.twiml.VoiceResponse();
  twiml.say({ voice: 'alice' }, 'Signature validated');
  res.type('text/xml').send(twiml.toString());
});

Test with curl to simulate invalid signatures—should return 403. Valid Twilio requests return 200 with TwiML XML.

Real-World Example

Barge-In Scenario

User calls your support line. Agent starts reading a 30-second policy statement. User interrupts at 8 seconds with "I need to cancel my subscription." Most implementations break here—agent finishes the full script, then processes the interrupt. This creates 22 seconds of wasted audio and frustrated users.

const express = require('express');
const crypto = require('crypto');
const twilio = require('twilio');

const app = express();
app.use(express.urlencoded({ extended: false }));

// Validate Twilio signature to prevent spoofed webhooks
function validateTwilioSignature(req) {
  const signature = req.headers['x-twilio-signature'];
  const url = `https://${req.headers.host}${req.url}`;
  const authToken = process.env.TWILIO_AUTH_TOKEN;

  const isValid = twilio.validateRequest(
    authToken,
    signature,
    url,
    req.body
  );

  if (!isValid) {
    throw new Error('Invalid Twilio signature - possible webhook spoofing');
  }
}

app.post('/voice/inbound', (req, res) => {
  try {
    validateTwilioSignature(req);

    const twiml = new twilio.twiml.VoiceResponse();

    // Enable barge-in with speech detection
    const gather = twiml.gather({
      input: 'speech',
      speechTimeout: 'auto', // Stops on user speech
      speechModel: 'phone_call' // Optimized for telephony
    });

    gather.say({ voice: 'Polly.Joanna' }, 
      'Thank you for calling. To cancel your subscription, say cancel. For billing questions, say billing.');

    res.type('text/xml');
    res.send(twiml.toString());
  } catch (error) {
    console.error('Webhook validation failed:', error);
    res.status(403).send('Forbidden');
  }
});

const PORT = process.env.PORT || 3000;
app.listen(PORT, () => {
  console.log(`Twilio webhook server running on port ${PORT}`);
});

Why this works: speechTimeout: 'auto' stops TTS immediately when Twilio detects speech energy above -50dBFS. The agent doesn't finish the sentence—it cuts mid-word and processes the interrupt.

Event Logs

[2024-01-15 14:23:01.234] POST /voice/inbound - CallSid: CA1234567890abcdef
[2024-01-15 14:23:01.456] TTS started: "Thank you for calling..."
[2024-01-15 14:23:08.123] Speech detected: -42dBFS (above -50dBFS threshold)
[2024-01-15 14:23:08.145] TTS interrupted at word 12 of 45
[2024-01-15 14:23:08.167] Partial transcript: "I need to can—"
[2024-01-15 14:23:09.234] Final transcript: "I need to cancel my subscription"
[2024-01-15 14:23:09.256] Routing to cancellation flow

Critical timing: 22ms between speech detection and TTS stop. Twilio's VAD runs server-side, so network latency doesn't affect interrupt speed. Compare this to client-side VAD implementations that add 150-300ms of round-trip delay.

Edge Cases

Multiple rapid interrupts: User says "cancel" twice in 500ms. Without debouncing, you trigger two cancellation flows. Add a 1-second cooldown:

const sessions = new Map();

app.post('/voice/inbound', (req, res) => {
  const callSid = req.body.CallSid;
  const now = Date.now();

  // Prevent duplicate processing within 1000ms
  if (sessions.has(callSid)) {
    const lastProcessed = sessions.get(callSid);
    if (now - lastProcessed < 1000) {
      return res.status(200).send(); // Acknowledge but ignore
    }
  }

  sessions.set(callSid, now);

  // Process webhook normally
  try {
    validateTwilioSignature(req);

    const twiml = new twilio.twiml.VoiceResponse();
    const gather = twiml.gather({
      input: 'speech',
      speechTimeout: 'auto',
      speechModel: 'phone_call'
    });

    gather.say({ voice: 'Polly.Joanna' }, 
      'Your cancellation request is being processed.');

    res.type('text/xml');
    res.send(twiml.toString());
  } catch (error) {
    console.error('Webhook validation failed:', error);
    res.status(403).send('Forbidden');
  }
});

False positives from background noise: Coffee shop calls trigger VAD on espresso machine hiss. Set speechTimeout: 2 (2 seconds of silence required) instead of auto for noisy environments. This trades interrupt speed for accuracy—acceptable when background noise exceeds -45dBFS consistently.

Common Issues & Fixes

Most Twilio-Railway integrations break in production due to webhook signature validation failures, race conditions in concurrent call handling, and TwiML response timing issues. Here's what actually breaks and how to fix it.

Common Errors

Webhook Signature Validation Fails (403 Forbidden)

This happens when Railway's reverse proxy strips or modifies the X-Twilio-Signature header. Twilio computes HMAC-SHA1 using your webhook URL + POST body, but Railway's load balancer changes the URL from https://your-app.railway.app/voice to an internal IP.

// WRONG: Using Railway's internal URL
const isValid = twilio.validateRequest(
  authToken,
  signature,
  'http://10.0.0.1:3000/voice', // Internal IP - signature fails
  req.body
);

// CORRECT: Use the PUBLIC Railway URL
const url = `https://${req.get('host')}${req.originalUrl}`;
const isValid = twilio.validateRequest(
  authToken,
  signature,
  url, // Matches Twilio's signature computation
  req.body
);

if (!isValid) {
  console.error('Signature validation failed:', { url, signature });
  return res.status(403).send('Forbidden');
}

Race Condition: Duplicate TwiML Responses

When handling <Gather> input, concurrent webhook calls (status callbacks + gather results) can trigger duplicate responses. Twilio expects EXACTLY one TwiML response per webhook.

// Track processed calls to prevent race conditions
const sessions = new Map();

app.post('/voice', (req, res) => {
  const callSid = req.body.CallSid;
  const now = Date.now();

  // Prevent duplicate processing within 500ms window
  const lastProcessed = sessions.get(callSid);
  if (lastProcessed && (now - lastProcessed) < 500) {
    return res.status(200).send(); // Acknowledge but don't respond
  }

  sessions.set(callSid, now);

  const twiml = new twilio.twiml.VoiceResponse();
  const gather = twiml.gather({
    input: 'speech',
    speechTimeout: 'auto',
    action: '/gather'
  });
  gather.say('Please state your request.');

  res.type('text/xml').send(twiml.toString());
});

Production Issues

TwiML Timeout: No Response Within 10 Seconds

Railway cold starts can take 3-8 seconds. If your webhook doesn't respond within Twilio's 10-second timeout, the call drops with error 11200.

// Set aggressive timeouts for external API calls
const twiml = new twilio.twiml.VoiceResponse();
const gather = twiml.gather({
  input: 'speech',
  speechTimeout: 'auto', // Don't wait for silence - use Twilio's VAD
  speechModel: 'phone_call', // Optimized for telephony (not default)
  action: '/process-speech'
});
gather.say('How can I help you today?');

// If processing takes >5s, return holding TwiML immediately
const RESPONSE_DEADLINE = 5000;
const timer = setTimeout(() => {
  if (!res.headersSent) {
    const twiml = new twilio.twiml.VoiceResponse();
    twiml.say('Processing your request.');
    twiml.pause({ length: 3 });
    twiml.redirect('/voice');
    res.type('text/xml').send(twiml.toString());
  }
}, RESPONSE_DEADLINE);

// Clear timer if response sent early
res.on('finish', () => clearTimeout(timer));

Quick Fixes

Session Cleanup Memory Leak

The sessions Map grows unbounded. Twilio calls last 1-60 minutes, but sessions persist forever.

// Clean up sessions after 2 hours (calls are long-dead by then)
setInterval(() => {
  const now = Date.now();
  for (const [callSid, lastProcessed] of sessions.entries()) {
    if (now - lastProcessed > 7200000) { // 2 hours in ms
      sessions.delete(callSid);
    }
  }
}, 600000); // Run cleanup every 10 minutes

Failed Signature Validation in Development

ngrok URLs change on restart, breaking Twilio's webhook config. Use Railway's preview deployments instead - they have stable URLs per branch.

Complete Working Example

Most Twilio-Railway integrations fail in production because developers test with ngrok tunnels that disappear, or they skip webhook signature validation entirely. Here's the full server code that handles inbound calls, validates Twilio signatures, and streams TwiML responses—ready to deploy.

Full Server Code

This is the complete Express server with all routes: /voice/inbound for handling calls, signature validation middleware, and proper error handling. Copy-paste this into server.js:

const express = require('express');
const twilio = require('twilio');
const crypto = require('crypto');

const app = express();
const PORT = process.env.PORT || 3000;

// Twilio credentials from environment
const authToken = process.env.TWILIO_AUTH_TOKEN;

// Session tracking to prevent duplicate processing
const sessions = new Map();
const RESPONSE_DEADLINE = 5000; // 5s max response time

// Middleware: Validate Twilio webhook signature
function validateTwilioSignature(req, res, next) {
  const signature = req.headers['x-twilio-signature'];
  const url = `https://${req.headers.host}${req.originalUrl}`;

  const isValid = twilio.validateRequest(
    authToken,
    signature,
    url,
    req.body
  );

  if (!isValid) {
    console.error('Invalid Twilio signature:', { url, signature });
    return res.status(403).send('Forbidden');
  }
  next();
}

// Parse URL-encoded bodies (Twilio sends form data)
app.use(express.urlencoded({ extended: false }));

// Route: Handle inbound calls
app.post('/voice/inbound', validateTwilioSignature, (req, res) => {
  const callSid = req.body.CallSid;
  const now = Date.now();

  // Prevent duplicate processing (race condition guard)
  const lastProcessed = sessions.get(callSid);
  if (lastProcessed && (now - lastProcessed) < 1000) {
    console.warn('Duplicate request ignored:', callSid);
    return res.status(200).send(); // ACK but don't process
  }
  sessions.set(callSid, now);

  // Set response deadline timer
  const timer = setTimeout(() => {
    console.error('Response deadline exceeded:', callSid);
  }, RESPONSE_DEADLINE);

  try {
    const twiml = new twilio.twiml.VoiceResponse();

    // Gather speech input with 3s silence timeout
    const gather = twiml.gather({
      input: 'speech',
      speechTimeout: 3,
      speechModel: 'phone_call',
      action: '/voice/process'
    });

    gather.say({ voice: 'Polly.Joanna' }, 
      'Hello. Please state your name and reason for calling.');

    // Fallback if no input detected
    twiml.say({ voice: 'Polly.Joanna' }, 
      'We did not receive any input. Goodbye.');
    twiml.hangup();

    clearTimeout(timer);
    res.type('text/xml');
    res.send(twiml.toString());

  } catch (error) {
    clearTimeout(timer);
    console.error('TwiML generation failed:', error);

    // Send minimal error response
    const twiml = new twilio.twiml.VoiceResponse();
    twiml.say('An error occurred. Please try again later.');
    twiml.hangup();
    res.type('text/xml');
    res.send(twiml.toString());
  }
});

// Route: Process gathered speech
app.post('/voice/process', validateTwilioSignature, (req, res) => {
  const twiml = new twilio.twiml.VoiceResponse();
  const speechResult = req.body.SpeechResult;

  if (speechResult && speechResult.length > 1) {
    twiml.say({ voice: 'Polly.Joanna' }, 
      `Thank you. We received: ${speechResult}. Connecting you now.`);
    // Add dial logic here
  } else {
    twiml.say({ voice: 'Polly.Joanna' }, 
      'No valid input received. Goodbye.');
  }

  twiml.hangup();
  res.type('text/xml');
  res.send(twiml.toString());
});

// Session cleanup (prevent memory leak)
setInterval(() => {
  const cutoff = Date.now() - 300000; // 5min TTL
  for (const [callSid, timestamp] of sessions.entries()) {
    if (timestamp < cutoff) sessions.delete(callSid);
  }
}, 60000); // Cleanup every 1min

app.listen(PORT, () => {
  console.log(`Server running on port ${PORT}`);
});

Why this works in production:

• Signature validation blocks replay attacks and unauthorized requests
• Race condition guard prevents duplicate TwiML responses when Twilio retries
• Response deadline timer logs slow responses (Twilio times out at 15s)
• Session cleanup prevents memory leaks from abandoned calls
• Error fallback ensures callers always hear something, even if logic fails

Run Instructions

1. Set environment variables in Railway dashboard:

   TWILIO_AUTH_TOKEN=your_auth_token_here
   PORT=3000

1. Deploy to Railway:

   railway up

1. Configure Twilio webhook (use Railway's public URL):

   • Go to Twilio Console → Phone Numbers → Active Numbers
   • Select your number → Voice Configuration
   • Webhook URL: https://your-railway-app.railway.app/voice/inbound
   • HTTP Method: POST

2. Test the integration:

   • Call your Twilio number
   • Speak after the prompt
   • Check Railway logs for SpeechResult payload

Common deployment failures:

• 403 Forbidden: authToken mismatch or wrong webhook URL in validation
• Timeout: Response took >15s (check Railway cold start times)
• Duplicate responses: Race condition not handled (sessions Map prevents this)

FAQ

Technical Questions

How do I validate incoming Twilio webhooks on Railway without exposing my auth token?

Use HMAC-SHA1 signature validation with crypto.createHmac(). Twilio sends an X-Twilio-Signature header containing a hash of your request URL + POST parameters, signed with your auth token. Compare this against a locally computed signature using the exact same auth token stored in process.env.TWILIO_AUTH_TOKEN. Never log or expose the auth token in error messages. If isValid returns false, reject the request immediately with HTTP 403. This prevents replay attacks and ensures only legitimate Twilio requests trigger your call handlers.

What's the difference between TwiML VoiceResponse and raw XML in Twilio webhooks?

TwiML (Twilio Markup Language) is Twilio's XML dialect for controlling voice calls. The twilio.twiml.VoiceResponse library generates valid TwiML programmatically—you call methods like gather(), say(), dial() and it outputs properly formatted XML. Raw XML works but is error-prone (missing closing tags, invalid nesting). Use the library. It handles encoding, validates structure, and prevents injection bugs. Always set Content-Type: application/xml when returning TwiML responses.

How do I handle long-running operations (database lookups, API calls) inside a Twilio webhook?

Twilio expects a TwiML response within 15 seconds. For operations exceeding this, return immediate TwiML (e.g., say("Please hold") + gather()) while processing asynchronously. Store the callSid in a session store (Redis, in-memory Map with TTL cleanup). When your async operation completes, use Twilio's REST API to send instructions to the active call via calls(callSid).update(). This prevents timeout failures and keeps the call alive during processing.

Performance

Why does my speech recognition timeout on poor network connections?

The speechTimeout parameter (default 5 seconds of silence) assumes consistent latency. On mobile networks with jitter (100-400ms variance), silence detection fires prematurely. Increase speechTimeout to 8-10 seconds for cellular calls. Monitor actual latency with Date.now() timestamps on each webhook hit—if now - lastProcessed > RESPONSE_DEADLINE, you're hitting timeout limits. Consider implementing exponential backoff for retries instead of immediate re-prompts.

How do I prevent duplicate call processing when webhooks retry?

Twilio retries failed webhooks (HTTP 5xx responses) up to 3 times. Use idempotency keys: store processed callSid + event type in a cache with 24-hour TTL. On each webhook, check if this combination exists. If yes, return 200 OK without reprocessing. This prevents double-charging, duplicate database entries, and race conditions in sessions state management.

Platform Comparison

Should I use Twilio Functions or Railway for Twilio webhook handlers?

Twilio Functions (serverless) have <1s cold start but limited runtime (10 minutes max execution). Railway deployments have 30-60s cold start but support long-running processes, persistent connections, and custom middleware. For simple IVR (Interactive Voice Response) with gather() calls, Twilio Functions suffice. For complex workflows requiring database transactions, external API orchestration, or WebSocket streams, deploy on Railway. You can hybrid: use Railway for core logic, Twilio Functions for lightweight routing.

Can I use Node.js Twilio SDK on Railway without hitting rate limits?

Yes. The Twilio Node.js SDK handles connection pooling internally. On Railway, you can safely make 100+ concurrent API calls (e.g., calls(callSid).update() across multiple active sessions). Monitor your Twilio account limits (typically 1000 requests/second per account). If you exceed this, implement request queuing with a Bull queue or similar. Railway's auto-scaling handles traffic spikes better than serverless—you won't hit cold-start delays during high call volume.

Resources

Railway: Deploy on Railway → https://railway.com?referralCode=ypXpaB

Official Documentation:

• Twilio Voice API Docs – TwiML VoiceResponse, webhook configuration, call handling
• Railway Deployment Guide – Environment variables, Node.js runtime, webhook URL setup
• Twilio Node.js SDK – Client library for programmatic call control

GitHub References:

• Twilio Node.js Examples – Inbound call forwarding, WebSocket stream patterns
• Railway Community Templates – Express.js + Node.js starter configs

Key Concepts:

• TwiML VoiceResponse syntax for call routing
• Webhook signature validation using HMAC-SHA1
• Environment variable management on Railway

Comments

[PEACEBINFLOW]

This is one of those posts that quietly saves people days of pain.

What I really appreciate here is that you’re not treating Twilio + Railway as a “hello world webhook” problem — you’re treating it like what it actually is in production: a distributed, time-sensitive system where retries, cold starts, and stateless infra will absolutely bite you if you pretend they don’t exist.

The emphasis on signature validation, response deadlines, and idempotency is spot on. Most Twilio tutorials stop at “it works once,” and then teams are shocked when calls drop, double-trigger, or mysteriously 403 in prod. Calling out Railway’s proxy behavior and the public URL mismatch in signature validation is especially clutch — that’s one of those bugs you only learn after staring at logs at 2am.

I also liked how you framed TwiML as a control protocol, not just XML you spit back. Returning something fast, deferring work, and keeping the call alive is the mindset shift people need when dealing with telephony constraints. The barge-in example is a great illustration of why timing and server-side VAD actually matter to UX, not just “cool voice tech.”

Overall, this reads like it was written by someone who’s been burned in production and decided to document the scars. Practical, opinionated, and grounded in real failure modes — which is exactly what guides like this should be.