They've enforced mobile numbers for quite a while now, though I suspect that might be country dependant. I can't sign up for an account without one, through my normal or torified IP (I tried several).
US based, for me, the form element appears, also with country code pre-filled, but it is not required. Further down there is a CAPTCHA that you can elect to complete or tick a box for phone verification. Strange that it's country dependent!
