Phone numbers can stop receiving text messages/calls for a million of other reasons, effectively either (1) locking users out of an account or (2) compromising the account altogether. There are alternatives where you can confirm your identity even if the device where you are authenticating has an Internet connection, and the “second factor” does not - apps like Authy, Microsoft Authenticator (yes, it supports standard tokens that are refreshed every 30 seconds) or Google Authenticator solve the problem singlehandedly. Or use U2F-compatible devices like the YubiKey which is probably the best way to go about secure authentication since it relies on a physical token be connected to the authenticating device.