How to Build Reliable SMS Login Flows That Handle Roaming and Delivery Failures

SMS-based login systems involve multiple independent layers — application, messaging provider, carrier routing, and device registration — any of which can silently fail, especially for roaming users. Developers are advised to model login code state granularly, distinguishing between provider acceptance, delivery confirmation, and verification, rather than relying on a single boolean flag. Phone numbers should be stored and transmitted in E.164 format, with country code and national number handled separately in the UI to prevent common input errors like duplicated prefixes or retained trunk digits. Delivery receipts, where available, should update attempt state, but their absence must be recorded as unknown rather than assumed successful. The article also recommends returning a correlation ID to users, keeping public error responses generic to prevent account enumeration, and reserving detailed failure data for authenticated support tooling.