Is it okay to ask 你的电话号码是什么?
I'd say all of your sentences are used in practice within the  mainland. 

These are all correct expressions: 


你的电话号码是什么？
你的电话号码是多少？
你的电话是多少？


Colloquially, we even brief it like 你电话多少？ or 电话多少？

Hope this could help you.