Is it a T-Mobile number?
Are you the primary account holder (the person paying the bill)?
If so, that number should definitely work.
Unfortunately no. the number that your iPad is - while it looks like a phone number is actually just an accounting number AND not a DIGITS. That is to say the number that your iPad has is just for data connectivity and doesn't have any voice capabilities -- DIGITS are all about taking talk and text (the core of your phone number) and letting them exist where ever you need them.
This thread was the product of our fantastic collaborators during DIGITS Beta! As many terms have changed and bugs have been exterminated since the Beta launched, the information here may no longer be accurate. If you still have a question, please search our active threads or ask a new question in the DIGITS space!