Why isn't SMS an option for two-factor authentication?

[komatta]

I want to set up an account when using the free ZOOM. I want to set up two-factor authentication, but why isn't the SMS option available?

[cky]

I have the same problem. It was originally working for years and it stopped working now. I tried to go into Zoom, after entering my password, the 6-digit code did not come by SMS on my phone. Had to use 1 of my recovery codes to get in to turn off 2FA. When I turn 2FA on again, I am not able to select SMS as an option. Why?