Well, MultiMC is open source so you can always disable the account check. I'm sure there's a patched fork already somewhere, however I'm not sure whether this is legal or not.

>Microsoft mines phone numbers

This seems to be more or less standard practice now. Happened to me with several companies. They let you sign up with just an email address, then after a few days or weeks they block your account because of "suspicious activity" and ask for a phone number. Very annoying.