I have not run into a case where the sole verification option offered (or at least mentioned) required one to provide a mobile phone number.
Is that really the case here?
In other words, does use of outlook.com email require one to own a cell phone -- even though that has nothing to do with sending or receiving email?
I am apparently being asked for this verification-by-mobile phone because I've attempted to send an email to eleven persons and that exceeds some unspecified limit for new users. Does Microsoft find it surprising that new users would have a contacts 'list' of eleven people?