Is it okay to ask 你的电话号码是什么? Is it ok to ask:

你的电话号码是什么? (What is your telephone number)

Or do I have to say:

你的电话号码是多少? which is the translation on Duolingo.

Also, can I just say:

&quot;你的电话是多少?

because 电话 already means phone number.
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.