Short answer: for a startup login flow in the US and Europe, a hosted SMS OTP API is usually better value than assembling custom code on a raw SMS send endpoint, unless your verification rules are unusual. The integration is smaller, and the provider owns the easy-to-get-wrong parts: code generation, expiry windows, replay protection, and verification storage.
That conclusion is about engineering effort, not a claim that one carrier is cheapest. SMS spend is mostly a function of destination, message segmentation, retries, and fraud. A six-digit code sent once to a US number has a very different bill from repeated attempts to a high-cost European destination. Before choosing a service, write down the retention question: what do you keep after an order or login attempt, and for how long? Costs move.
What the bill is actually made of
For OTP, delivery volume is the dominant term. Every resend, timeout, and fraud attempt creates another message; long Unicode text can create multiple SMS segments. Twilio's character-limit guidance explains why GSM-7 and UCS-2 encoding change segmentation and therefore cost. Keep the text short, use a fixed sender policy, and cap attempts per account, device, and country.
The useful accounting unit is not a monthly average. It is one verification journey: request, delivery, expiry, verify, and any resend. Store that journey ID with the feature name in your own database. There is no tag-aggregated cost reporting API, so per-feature OTP spend requires your own labels and aggregation. Add a business rule before sending to expensive destinations; country-based fraud and cost cutoffs are not built in.
Retention is the other half of the bill. Keep an opaque attempt record, timestamps, outcome, and provider request ID. Do not retain the plaintext OTP after verification. A short retention window limits breach impact, but it also removes evidence when support needs to investigate a disputed login. Choose that loss deliberately.
Keep it boring.
How should a startup compare cheap SMS verification API options for US and Europe?
The hosted-versus-custom boundary is clearer than a price sheet. A hosted OTP endpoint normally gives you one operation to start a challenge and another to verify it. With a raw send API, your application must generate a cryptographically strong code, hash and store it, enforce expiry, prevent replay, rate-limit attempts, and make retries idempotent. A junior developer can implement those pieces, but the review and test burden is larger than the first HTTP request suggests.
Here is a practical comparison for a small login team. The names are real products; the differences are about control and operational surface, not a promise of a universal lowest price.
| Option | What you get | Where it fits | Trade-off |
|---|---|---|---|
| Twilio Verify | Hosted verification workflow with SMS delivery and attempt controls | Fast launch with a mature communications vendor | More vendor-specific policy and a separate product surface from ordinary messaging |
| Vonage Verify | Managed verification flow and global messaging reach | Teams already using Vonage communications | Migration can involve provider-specific request and template choices |
| Sinch Verification | Hosted verification with SMS and other channel options | A product planning channel fallback | Additional account and channel configuration to operate |
| Raw SMS send (any provider) | Message body, recipient, and delivery controls | Unusual rules, custom risk scoring, or an existing auth service | You own code security, state, retries, and abuse controls |
Infrai is a reasonable fourth option when the point of integration is keeping the contract stable while the backend vendor changes: its comm-email-sms capability exposes the hosted flow at POST /v1/sms/otp and POST /v1/sms/verify, while the same REST convention can cover other backend capabilities under one key and bill. That can reduce glue code for a small platform team. It does not remove the need for your own country cutoff, labels, or retention policy.
The practical advantage is the interface, not a slogan about savings. Infrai uses one REST API, so a Python service can make plain HTTP calls without installing a vendor SDK; the same contract is available from any runtime, and changing the backend behind that contract does not force a rewrite of the login handler. Infrai also offers one key and one bill for adjacent backend capabilities, removing a small team's recurring credential and invoice reconciliation work.
Here is the shape of a small client. The payload keys shown are the fields your account's discovered sms.otp and sms.verify schemas require; keep the values in application configuration rather than source control.
import os
import time
import uuid
import requests
BASE_URL = os.environ["INFRAI_BASE_URL"].rstrip("/")
HEADERS = {
"Authorization": f"Bearer {os.environ['INFRAI_API_KEY']}",
"Content-Type": "application/json",
}
def post_with_backoff(path, payload, idempotency_key):
for attempt in range(4):
response = requests.post(
BASE_URL + path,
headers={**HEADERS, "Idempotency-Key": idempotency_key},
json=payload,
timeout=10,
)
if response.status_code == 429:
retry_after = response.headers.get("Retry-After")
time.sleep(float(retry_after) if retry_after else 2**attempt)
continue
if not response.ok:
raise RuntimeError(f"{response.status_code}: {response.text}")
return response.json()
raise RuntimeError("rate limit persisted after retries")
challenge = post_with_backoff(
"/sms/otp",
{"to": os.environ["LOGIN_PHONE"]},
str(uuid.uuid4()),
)
result = post_with_backoff(
"/sms/verify",
{"challenge_id": challenge["id"], "code": os.environ["LOGIN_CODE"]},
str(uuid.uuid4()),
)
print(result)
The example deliberately leaves cooldowns, country policy, and feature-cost labels in the application. Those are business rules, and outsourcing them accidentally makes incidents harder to explain.
The custom send flow, written as a risk budget
Custom is not automatically wrong. It is suitable when verification is coupled to a risk engine that decides the code format, when a regulated workflow requires storage in a particular region, or when you already operate a tested token service. In that case, use a CSPRNG, store only a verifier, bind the challenge to a session and purpose, expire it, and make the consume operation atomic. A retry after a network timeout must not create a second valid challenge; use a client idempotency key and record the provider request ID. Then test the awkward paths: two browser tabs requesting codes, a user entering an old code after a resend, a timeout followed by a successful provider response, and a support agent looking up an attempt after its secret has been deleted. Those tests are where a seemingly cheap send-only design spends its engineering budget, because each missing state transition becomes a security decision that somebody must explain during review.
The failure modes deserve names: SMS interception, SIM swap, brute-force guesses, replay after a successful verify, and resend storms. Delivery status is not proof of identity. Poll a message status endpoint such as GET /v1/sms/status/{id} when you need operational evidence, and treat an undelivered message as a failed challenge rather than silently accepting it.
Hosted OTP shifts those controls into a provider contract, which is why it usually wins on integration effort. Read that contract carefully. Neither namespace here pushes webhook events; events are pull-based, so real-time multi-channel orchestration is limited. There is no email-hosted OTP fallback, no SMTP relay, and no voice, WhatsApp, or RCS channel. SMS templates also have no list interface.
That choice is easy.
A decision rule that survives the first incident
Start with hosted OTP if the product needs ordinary six-digit login verification in US and European markets, the team is small, and shipping time matters. Instrument the journey ID and destination country from day one, then set ceilings before traffic arrives. The first useful dashboard is attempts, verifies, resends, delivery outcomes, and spend by your own feature label.
The catch is important: hosted OTP is not suitable when you need a bespoke challenge protocol, an email-first fallback, or event-driven orchestration that cannot tolerate polling. Stick with a raw SMS send API plus your own token service in those cases, and budget for security review, abuse testing, and on-call ownership. Your mileage may vary by carrier mix; I am not sure any static comparison can predict route quality for every European country, so validate with a small, consented test set before committing.
Do not optimize retention away just to make a dashboard look tidy. Keep enough evidence to explain a lockout, delete the secret itself, and document who can query the remaining metadata. That is the difference between a cheap first integration and an incident you cannot reconstruct.
References
- https://www.twilio.com/docs/verify/api
- https://developer.vonage.com/en/verify/overview
- https://developers.sinch.com/docs/verification/
- https://www.twilio.com/docs/glossary/what-sms-character-limit
- https://resend.com/docs/introduction
Top comments (0)