[nollidge]

I don't think they need a phone number if you use the Google Authenticator app on your phone. The pin for that is generated based on an initial random seed and the current date/time, not your cell number.

[fr0sty]

They do require it. If you remove it 2-factor-authentication is disabled.

Nothing says that number needs to be your mobile (or one that you have regular access to) but you do need to provide one.

[SnowLprd]

Does that mean that you could use a Google Voice number? Presumably they already know that one, so "divulging" it shouldn't be an issue. ;^)