it's not, it asks for one but you can omit it, even though it will keep bugging you about it from time to time.

It didn't when I signed up, and thus far, I've not been forced to add it.

Perhaps that's a new requirement. Some of us don't bother with mobile phones. I signed up for an account, at one time it asked, another it didn't. Try and empty your browser cache and try again.