I feel like I've had my say on this thread. Please indulge me one more post:
Firstly I'm not a MS hater that is just finding fault. I like what I know about Azure. When I went to sign up I had my wallet on my desk I was ready to fork over some bucks. I am a MS developer. I want to learn Azure. I want to use it.
The flatulence about two factor auth is a red herring and not the subject of this thread. A phone is not any more of a security device than a laptop or a desktop or a wristwatch or an abacus. BTW please note that MS can, and does, obtain hardware ID's and various software ID's from computers and phones. This is only one of dozens of reasons why MS does not need to REQUIRE your phone number.
Again - MS does not need to require you to give them your phone number. And, furthermore, if you choose to give it to them you should be protected by TOS which gives you ironclad protection against misuse or third party disclosure. Its not rocket science guys.
Several have mentioned that MS has been good about not spamming. Generally, I agree. However: Don't expect that in the future. I got so much spam for windows store that I had to take time to "opt out". Do you think I ever "opted in?" Seriously? Did I unsubscribe from every affiliate that bought my info? Probably not.
You don't know what you don't know. Remember that MS has the "affiliate" clause which means they can sell your info to whoever wants to buy it. So you may have been spammed by an affiliate and not known they got your info from MS.
Finally, if your phone is like mine you dont have a junk box for spam sms messages. And if the big players get their way, you never will. Your phone is going to become a toilet for spammers and it is going to become as useless as that hotmail account you never use anymore........because you get so much spam that its impossible to use.
Its not going to happen overnight. It begins with small baby steps, just like what you see with Azure.
Next, you will need to give MS your phone number to use msdn.
Then windows support forums.
Then windows update.
Next you will see messages on your phone to the effect "The icon for notepad has been updated!!!!"
Later you will see "The icon for notepad has been updated!!! Refinance your mortgage now...yada"
I mention MS in this post, but Google is equally guilty of intrusion, if not more so. Everything I've said applies to them also.