TOTP (time-based one-time password)

A one-time code generated on your device from a shared secret and the current time, used by authenticator apps like Google Authenticator.

TOTP codes are computed locally: the service and your authenticator app share a secret at setup, and both derive the same 6-digit code from that secret plus the clock, typically rotating every 30 seconds. No SMS, no network, nothing to intercept in transit.

For accounts verified with a rented number, TOTP is the recommended second factor precisely because it removes the phone number from the account's ongoing security.