https://www.fastmail.com/help/account/2fa.html

>> Why do I have to add a recovery phone number to set up two-step verification?

> Keeping your account safe from attackers is very important. But so too is making sure you don't get locked out of your own account. We are aware that SMS is not the most secure of methods for 2FA, and has been deprecated by NIST. However, for the majority of users, the risk of losing their two-step verification device is far greater than the risk of someone hacking their SMS. If you lose your phone, the TOTP key is lost but normally you can get a new SIM card with the same number from your carrier. We therefore believe requiring a phone as a backup option strikes the best balance of confidentiality (no one else can read your data) and availability (you can read your data) for the majority of our users.

> Please note, if two-step verification is enabled, access to the phone number itself is not sufficient to gain access to an account: you still need two factors (your password AND the SMS).

> Advanced users that understand the risk may remove the phone number from their account once two-step verification is enabled. Once the recovery phone is removed from the account, SMS is no longer an option as the second factor for login. If you choose to do this, we strongly recommend you write down or print your recovery code and store it in a safe location, and that you set up at least two security keys or authenticator devices. Should you lose access to all two-step verification devices and not have your recovery code, you may be permanently locked out of your own account.