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What else can TFA fall back on. I know steam uses email and iOS tells you're other devices, but is there another option.

TOTP[1] is a common one. I use the FreeOTP[2] Android app but there are plenty of other options.

* Pre-printed codes (like in Google)

* Trusted friends (like in Facebook)

* Renew your password in the office (like in my university)

* User-side SSL certificate

* Oh, end then Telegram introduced PIN+TFA