On the iPad you'll need to log in with your T-Mobile ID. Your contacts that you uploaded are stored with your ID and not the phone number, so when you log in via "use my number" it doesn't know which ID to get the contacts from since you'll be able to allow multiple people to use that number (at a later date).
Probably not exactly the answer you're looking for, but unfortunately that's how the contacts are currently designed.
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!