Designing Timeout and Retry for SMS Verification APIs: First Distinguish Request Timeout from SMS Not Arriving

In SMS verification APIs, "timeout" means two things: API communication timeouts require jittered exponential backoff and idempotent retries, while SMS reception timeouts require polling cadence, a total wait window, and explicit order finalization. This article provides actionable polling intervals, wait windows, cancellation timing, and a tiered retry strategy with circuit breaker limits: same-country number change → country change → switch to a real carrier long-term number.