SMS verification that actually arrives: a practical checklist
Delivery rate, latency, and fallback strategy — everything product teams should tighten before scaling OTP traffic.
Why OTP delivery rate matters more than price
A 2% delivery gap on verification SMS translates directly to 2% of signups that never complete. Over a year, that's usually a multiple of what you'd save by picking the cheapest carrier. The right question isn't 'how cheap is your SMS?' — it's 'what does your median delivery time look like on verification routes, and what's your p99?'
The four levers that move delivery
1) A registered sender ID (alphanumeric or short code) in every country you send to. 2) A verification-grade route — one with direct carrier agreements, not a re-seller's backup route. 3) A sensible code format — 6 digits, no branding, short body. Most spam filters trigger on long bodies with mixed content. 4) Automatic failover: if the primary route dips below your threshold in the last 5 minutes, retry on a backup route with a different carrier.
What to monitor
Per-route delivery rate (rolling 1h + 24h), median delivery time, p95 and p99 delivery time, and opt-out rate even on transactional traffic. Opt-out isn't legally required for OTP, but if your users STOP on verification messages, something is wrong — often a caching or reuse bug sending the same code multiple times.
Common mistakes
Reusing the same route for marketing and verification (carriers throttle your verification traffic if your sender becomes associated with spam). Not honoring STOP keywords (not just illegal — it tanks delivery scores). Sending verification codes with links in the body (many carriers filter).
A minimal checklist
Registered sender ID per destination country · Verification-tier route with failover · 6-digit numeric code, 10-minute TTL, single-use · Rate limit per phone number (3 codes / 10 min is reasonable) · Monitor delivery rate and alert below 97% · Log every send with route and latency for post-mortem analysis.
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