I hate the tendency to demand phone number to activate two factor. I see no practice reason why not simple TOTP isn't good enough. It's probably not a practical reason though...
Probably want phone as fallback auth for the inevitable people who lose their TOTP device and didn't save any emergency codes.
That could be setup as an option. The fact that there is no option provided and a phone number is mandatory means that they want to be able to track people with a globally unique identifier.