Model a situation: when two-factor authentication SMS code does not arrive. What could be points of failure?

In two-factor authentication, when the SMS code does not arrive, the following points of failure are possible:

1. User-side issues:

   • Incorrect phone number or format.
   • Phone out of network coverage or in "airplane mode".
   • SMS blocking on the device (e.g., spam filters).

2. Carrier issues:

   • Network failures of the carrier.
   • Delays or blocks of SMS messages.

3. SMS sending system issues:

   • Failure in the SMS sending service (e.g., provider API).
   • Exceeding message sending limits.
   • Errors in integration with the SMS gateway.

4. Authentication server issues:

   • Errors in code generation or sending.
   • Timeouts or failures in request processing.

For diagnostics, it is important to check system logs, SMS gateway status, the correctness of the number, and the user's network condition.