Unable to register my new phone number on WhatsApp

Your number should work for What's App unless they have a block/ban on your number.  What is the issue though?  Is What's App refusing to take the number or are you not receiving a verification text from them to confirm your number?  If it is the text, you will need to reach out to Customer Care and make sure that Short Codes are enabled on your account and line.  You can also reach out to T-Mobile via DM on Facebook or Twitter to accomplish the same thing.