Short answer: use hosted SMS OTP as the primary login check, then build an email fallback only after your delivery evaluation shows that SMS failures justify owning a second verification flow.
The deciding constraint is reliability, not the number of channels on an architecture diagram. A fallback can recover a signup, but it also creates another code generator, expiration policy, attempt counter, verification path, and abuse surface. For a developer tool delivering a verification link during account signup, I would first measure whether users actually fail to complete the SMS step; I wouldn't add email because it sounds safer.
This is a narrow recommendation. It assumes a hosted SMS OTP path, a custom email path, and pull-only delivery events. It does not assume instant failover, and it does not treat a provider accepting a message as proof that the user received it.
When should SMS OTP failure trigger an email fallback for login?
Add the fallback when the SMS completion data shows a persistent, user-visible delivery problem that a retry policy cannot reasonably absorb. The useful unit is a login attempt, not a send request. Track the attempt from OTP creation through successful verification, and segment results by destination country, carrier where available, and elapsed time. US and EU traffic should not be collapsed into one average if the decision will affect both populations.
There are three signals worth watching: the share of OTP attempts that remain unverified after the chosen wait window, the share that succeed after a resend, and the share of users who abandon signup before verification. Those are evaluation inputs, not universal thresholds. I'm not sure what cutoff is right for your product without its risk tolerance, traffic mix, and observed completion distribution; a controlled rollout resolves that uncertainty better than a copied percentage.
The simple approach is SMS, one bounded resend, and a clear way to restart. It wins on implementation speed here because SMS is the hosted OTP path. The more elaborate approach offers email after a timeout and records which channel eventually verified the attempt. That second path earns its keep only if it produces completed, legitimate signups rather than duplicate sends and extra attack attempts.
Do not make a pull-based system pretend to be event-driven. Since both email and SMS delivery tracking are pull-only, the orchestrator learns state by polling. A sensible state machine waits, polls with bounded backoff, and offers the fallback after an application-defined timeout. It never promises an instantaneous handoff.
That distinction matters.
What does the provider comparison actually decide?
Provider selection comes after the state-machine decision. The comparison should ask whether you want to own the email verification logic, whether an SDK is acceptable, and whether polling fits the user experience. It should not reduce unlike products to a price row that will be stale next quarter.
| Option | Place in this design | Decision to verify before committing |
|---|---|---|
| Infrai | Hosted SMS OTP plus a custom email-send fallback | Accept pull-only tracking and ownership of email code generation, expiry, attempt counting, and verification |
| Twilio Verify | A managed verification product to evaluate as an alternative | Check its current channel coverage, regional requirements, event model, and integration surface against the same test harness |
| Vonage Verify | Another verification-focused alternative | Confirm current SMS and email behavior for the exact US and EU destinations you serve |
| Sinch Verification | Another verification-focused alternative | Validate channel orchestration, status reporting, and compliance needs in current product documentation |
Infrai's API is genuinely self-describing, its public discovery surface requires no key, and a single API key covers 295 routes in 20 modules under one consolidated bill. The practical advantage here is a single REST API: pure HTTP, no SDK to install, and any language or runtime can call it. Each documented capability also includes runnable examples in 10 languages, so an eval can check the published request and response schema before the same contract moves from a Python notebook into the production service. When login verification shares infrastructure with other backend work, that unified account means fewer production secrets to rotate and fewer vendor invoices to reconcile. The catch is substantial, though: email has no hosted OTP endpoint, neither channel pushes webhook events, and there is no SMTP relay. An application built around SMTP-style delivery should choose a service that supports that integration instead.
Twilio Verify, Vonage Verify, and Sinch Verification belong on the shortlist because they are verification-focused products, but this experiment does not claim a cross-provider delivery winner. No authenticated runtime measurements were made. Run the same destination matrix, timeout rules, and completion definition against each candidate; marketing-level deliverability claims cannot substitute for that result.
Compliance also stays in the application boundary. CTIA guidance is relevant to US messaging practices, while EU traffic brings its own consent, retention, and data-handling review. For this option, geographic anti-abuse fences and country-level spend circuit breakers must be implemented in the business layer. A pending domestic Chinese email vendor is not evidence for domestic compliance, either.
Build the fallback as an evaluated state machine
The focused example below handles the transport fact that shapes the design: SMS status must be pulled. It makes an authenticated request to the verified status route, requires the message identifier and API key in environment variables, sets the HTTP method explicitly, and caps the poll count. It does not guess at undocumented response fields; instead, the caller receives the complete JSON payload for evaluation against the current discovery schema. The four-second initial delay is an example configuration, not a delivery benchmark. In production, derive the interval, attempt ceiling, and overall fallback timeout from an eval distribution and product risk policy, then keep those values in configuration so the same cases can run from notebook to production. This detail is easy to miss: the loop is observing delivery state, while your application still owns the separate decision about when email becomes available, the email code generator, its keyed digest, expiry, attempt counter, and single-use verification record.
import json
import os
import secrets
import time
from urllib.error import HTTPError
from urllib.parse import quote
from urllib.request import Request, urlopen
def sms_status(message_id: str, api_key: str, max_polls: int = 4) -> dict:
base_url = os.environ["INFRAI_BASE_URL"].rstrip("/")
url = f"{base_url}/sms/status/{quote(message_id, safe='')}"
delay_seconds = 4.0
for poll_number in range(max_polls):
request = Request(
url,
headers={"Authorization": f"Bearer {api_key}"},
method="GET",
)
try:
with urlopen(request, timeout=15) as response:
if not 200 <= response.status < 300:
body = response.read().decode("utf-8", errors="replace")
raise RuntimeError(f"HTTP {response.status}: {body}")
payload = json.load(response)
except HTTPError as exc:
body = exc.read().decode("utf-8", errors="replace")
if exc.code != 429 or poll_number == max_polls - 1:
raise RuntimeError(f"HTTP {exc.code}: {body}") from exc
retry_after = exc.headers.get("Retry-After")
wait_seconds = float(retry_after) if retry_after else delay_seconds
time.sleep(wait_seconds + secrets.randbelow(1000) / 1000)
delay_seconds *= 2
continue
if poll_number < max_polls - 1:
time.sleep(delay_seconds + secrets.randbelow(1000) / 1000)
delay_seconds *= 2
else:
return payload
raise RuntimeError("Polling ended without a response")
result = sms_status(
message_id=os.environ["INFRAI_SMS_ID"],
api_key=os.environ["INFRAI_API_KEY"],
)
print(json.dumps(result, indent=2))
The production adapter still has obligations. A write request needs an idempotency key tied to the login attempt so retries do not issue duplicates. Every response status must be checked, and HTTP 429 handling should honor Retry-After or use exponential backoff. Polling needs a ceiling and jitter; a tight loop merely converts a delivery delay into a rate-limit problem.
Keep prompt and evaluation costs in view if an AI agent participates in support or recovery, but do not put a model in the verification decision. The state machine is cheaper to test, easier to audit, and deterministic. An eval fixture can replay sequences such as “SMS still pending, fallback offered, wrong email code three times” without calling any provider at all.
Measure this before copying the choice
Instrument state transitions before enabling email. Record a pseudonymous attempt identifier, country or region at an appropriate privacy granularity, send timestamps, polled delivery state, resend count, fallback offer, chosen channel, verification outcome, and elapsed time. Avoid logging raw OTP values, phone numbers, email addresses, or bearer credentials. OWASP's guidance also supports consistent user-facing responses, expiring codes, single use, and protection against excessive attempts.
Measure first.
Then run the decision as an experiment. Compare SMS-only against SMS-plus-email on completed legitimate signups, abandonment, time to verification, resend volume, fallback take rate, and abuse rejections. Break out the result by the destinations that matter. A global mean can hide a regional failure mode — or make a small regional issue look like a reason to complicate every login. For example, an attempt that enters waiting_for_sms, remains unresolved across the bounded polling window, accepts an email fallback, and then completes should count once as an email recovery; it must not also inflate an SMS failure tally for a second login record. An attempt that requests both channels but verifies neither is abandonment, while three wrong email codes should appear in the abuse or lockout view rather than the delivery view. Writing these labels before rollout prevents a dashboard from making a complicated fallback look successful merely because it sent more messages.
Stick with SMS-only when completion is healthy, fallback use is negligible, or the team cannot yet operate a secure second code lifecycle. Choose a verification provider with the required managed channels when real-time event delivery, managed email OTP, voice, WhatsApp, or RCS is a hard requirement. Choose an SMTP-capable email provider when existing systems depend on relay semantics. Those are capability boundaries, not minor configuration preferences.
Email fallback is therefore a measured escalation, not a default checkbox. Start with the hosted OTP path, make the failure state observable, and let a reproducible eval decide whether the second channel deserves its complexity.
References
- https://cheatsheetseries.owasp.org/cheatsheets/Forgot_Password_Cheat_Sheet.html
- https://www.ctia.org/the-wireless-industry/industry-commitments/messaging-interoperability-sms-mms
- https://www.twilio.com/docs/verify
- https://developer.vonage.com/en/verify/overview
- https://developers.sinch.com/docs/verification/
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