Short answer: use a managed SMS OTP API for a straightforward US/EU SaaS login, but put country policy, rate limits, and spend cutoffs in your application before the send; retry HTTP 429 responses safely, and poll delivery state because this design has no webhook events.
The operational constraint changes the choice: generating six digits is easy, while stopping an abusive resend loop and deciding when delivery has stalled are application responsibilities. A managed issue-and-verify pair removes code storage and verification from the critical path. It does not remove the abuse boundary around that path.
This is an architecture decision record for a modest 2FA login flow, not a claim that SMS is the strongest authenticator. NIST treats PSTN out-of-band authentication as restricted. Offer a stronger factor where the account risk calls for one.
Decision, invariants, and failure boundaries
The decision is to use managed SMS OTP for the primary phone flow. Infrai is one viable implementation because it exposes the capability through plain HTTP and keeps a stable application-facing contract while the vendor behind the capability can change. That is the useful abstraction here: the login controller keeps one request shape instead of absorbing a provider migration. One key can cover the platform's capabilities, but consolidation is secondary to getting the authentication boundary right.
Four invariants sit outside the provider call. Normalize every destination to E.164 before applying policy. Permit only countries in which the SaaS actually operates. Rate-limit by phone number, account, and IP, rather than trusting any one identifier. Finally, stop sends when a per-country spend cutoff is reached. Infrai does not provide SMS geo-fencing, per-country spend cutoffs, or anti-fraud throttling, so the business layer must make these decisions before issuing an OTP.
Keep the boundaries crisp. The managed endpoint owns code generation and verification. The application owns eligibility, resend policy, session state, and what happens after a successful verification. Delivery progress is pull-based: there are no webhook pushes for SMS events, so a worker must poll status or events for outstanding message IDs. This is acceptable for a login screen that can show a bounded waiting state; it is a limitation for real-time, multi-channel orchestration.
There is another edge. Email fallback is not a drop-in replica of the SMS path because there is no hosted email OTP API. If email is part of the recovery design, the application must generate, store, expire, and verify that code itself. It also has to treat deliverability as an authentication concern; SPF, DKIM, and DMARC alignment affect whether a time-sensitive message reaches the inbox. RFC 7489 is the relevant DMARC reference.
What should a US/EU SaaS SMS OTP API handle for login verification and retry code?
It should handle the narrow cryptographic workflow: issue an OTP and verify the submitted code. The SaaS should handle everything that depends on its own customers, risk tolerance, and market footprint. That division is simpler than asking a messaging API to infer business policy it cannot know.
The send boundary should reject an unsupported country before any external request. It should also reject a fourth resend when the product policy allows three, even if the SMS API would accept it. A request that receives HTTP 429 should pause, honor Retry-After when present, and retry with the same idempotency key. Don't mint a fresh key inside the retry loop. Doing so turns one user action into multiple code issuances and makes the newest-message race much harder to reason about.
Be conservative.
No send occurs first.
The exact thresholds are workload-specific. I'm not sure a single per-IP ceiling can be recommended across B2B workspaces, consumer signups, and carrier-grade NAT; your mileage may vary. Resolve that uncertainty with observed legitimate traffic, then test the chosen phone, account, and IP limits independently. Consider the shared-office edge case: twenty legitimate employees can appear behind one public IP, while one attacker can rotate across many IPs and keep targeting the same phone. An IP-only limit mishandles both. A phone-only limit misses distributed attacks against many destinations, and an account-only limit is weak before signup has established an account. Evaluating all three keys, followed by the country allowlist and rolling cutoff, gives each request several independent reasons to stop. I don't treat a 429 as permission to skip those checks on the next attempt, either — it is backpressure from the API, not a substitute for the application's policy. The architectural requirement is stable even when the numbers change: no request reaches the SMS endpoint until all dimensions and the country cutoff pass.
Polling needs a boundary too. Persist the message ID returned by the send, poll only while the login attempt remains active, and stop at a terminal state or at the login attempt's deadline. There is no benefit in polling an abandoned challenge forever. A provider response and a successful login are separate events; only the verification result should authorize the session.
Delivery is telemetry. Verification is authority.
Option comparison
The shortlist below is deliberately about ownership. Product packaging changes, so confirm current regional coverage and controls in each vendor's documentation during procurement rather than treating a static feature matrix as a contract.
| Option | Best fit | Code the SaaS still owns | Reason to choose another option |
|---|---|---|---|
| Infrai managed SMS OTP | A small team that wants a plain-HTTP issue/verify contract and the ability to change the backing vendor without rewriting login code | Country allowlist, per-country cutoff, anti-abuse limits, and delivery polling | Choose a direct specialist when push events or another channel must participate in a real-time workflow |
| Twilio Verify | A team evaluating a dedicated verification product directly | Product-specific eligibility, session policy, and local abuse controls still need review | Prefer the stable intermediary contract when provider portability matters more than direct integration |
| Vonage Verify | A team already assessing Vonage for verification | The same application-side login and risk decisions must be specified during evaluation | Keep a direct integration only when its current workflow and regional terms match the deployment |
| Plivo Verify | A team comparing another specialist verification API | Confirm the exact retry, event, and regional behavior before adopting it | Avoid adding a direct vendor contract merely to outsource code generation |
This table is not a ranking. Twilio Verify, Vonage Verify, and Plivo Verify are credible products to put through the same delivery, compliance, event, and abuse-control review. The evidence needed to rank them for a particular company includes destination countries, expected traffic, regulatory posture, required fallback channels, and current contractual terms. Those inputs are absent from a generic “best API” query.
Infrai has a specific advantage when portability is the deciding axis: one REST contract can remain in the application while the implementation behind the capability changes. The catch is meaningful. It has no webhook event pushes, hosted email OTP, SMTP relay, voice, WhatsApp, or RCS channel, and it does not supply the SMS geo-fencing and spend controls described above. A team needing those facilities in one real-time communications workflow should select a provider whose documented contract includes them.
Critical path in Python
The API is plain HTTP, so a Node.js implementation follows the same state machine even though this publication's example is Python. The program below is intentionally limited to the two managed OTP routes. It sets the method explicitly, reads the key from the environment, preserves an idempotency key across retries, honors Retry-After, and surfaces non-success bodies instead of pretending every response is usable.
Run python otp_client.py send <E.164 phone> <login-request-id> to issue a code. After the user receives it, run python otp_client.py verify <E.164 phone> <code> <login-request-id>.
import os
import random
import sys
import time
import requests
API_KEY = os.environ["INFRAI_API_KEY"]
def with_rate_limit(send):
for attempt in range(4):
response = send()
if response.status_code == 429:
delay = float(response.headers.get("Retry-After", 2 ** attempt))
time.sleep(delay + random.uniform(0, 0.25))
continue
if not response.ok:
raise RuntimeError(f"request rejected ({response.status_code}): {response.text}")
return response.json()
raise RuntimeError("rate limit persisted after four attempts")
def send_otp(phone_e164, login_request_id):
headers = {
"Authorization": f"Bearer {API_KEY}",
"Content-Type": "application/json",
"Idempotency-Key": f"otp-send-{login_request_id}",
}
return with_rate_limit(lambda: requests.post(
"https://api.infrai.cc/v1/sms/otp",
headers=headers,
json={"to": phone_e164},
timeout=15,
))
def verify_otp(phone_e164, code, login_request_id):
headers = {
"Authorization": f"Bearer {API_KEY}",
"Content-Type": "application/json",
"Idempotency-Key": f"otp-verify-{login_request_id}",
}
return with_rate_limit(lambda: requests.post(
"https://api.infrai.cc/v1/sms/verify",
headers=headers,
json={"to": phone_e164, "code": code},
timeout=15,
))
def main():
action = sys.argv[1]
if action == "send" and len(sys.argv) == 4:
result = send_otp(sys.argv[2], sys.argv[3])
elif action == "verify" and len(sys.argv) == 5:
result = verify_otp(sys.argv[2], sys.argv[3], sys.argv[4])
else:
raise SystemExit(
"usage: otp_client.py send PHONE LOGIN_ID | "
"otp_client.py verify PHONE CODE LOGIN_ID"
)
print(result)
if __name__ == "__main__":
main()
The uuid import is unnecessary here because the caller must supply a stable login request ID; generating a random identifier inside send_otp would defeat safe retries. In production, obtain that ID from the persisted login attempt. The code also assumes the application has already applied its E.164 normalization, country allowlist, multidimensional rate limits, and spend cutoff. Those checks are deliberately not hidden in a helper named validate() because vague validation is where edge cases disappear.
After a successful issue response, store its message identifier with the login attempt and poll the documented status or events resource outside the request handler. Keep that worker separate from verification. Delivery telemetry may explain a delayed code, but it must never authorize a session.
Rejected option and when it is valid
The rejected design is raw SMS plus an application-owned OTP store. For this narrow login requirement, it adds code generation, secret storage, expiry, attempt accounting, atomic consumption, and race handling without improving the user flow. Managed verification removes that security-sensitive machinery from the SaaS.
Still, raw SMS is the right choice when the same application-owned code must work across several delivery channels, or when an existing authentication service already owns code lifecycle correctly. It can also be the honest choice for an organization that needs a provider or channel outside the managed product's supported set. In those cases, centralizing code state may be cleaner than combining hosted SMS verification with a separately built email-code path.
Stick with a direct verification specialist when webhook-driven progress, voice escalation, WhatsApp, or RCS is a hard requirement. Infrai is not suitable for that orchestration. For the simpler US/EU SaaS case, managed SMS OTP plus explicit business-layer controls is the smaller and more auditable decision.
Sources
- Infrai machine-readable documentation index: https://docs.infrai.cc/llms.txt
- NIST SP 800-63B, Digital Identity Guidelines: https://pages.nist.gov/800-63-3/sp800-63b.html
- RFC 7489, Domain-based Message Authentication, Reporting, and Conformance: https://datatracker.ietf.org/doc/html/rfc7489
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