Depends on what you want to do.
You can sign into the DIGITS app on any phone (any phone that can run the app) and use any number 'attached' to your account.You just need to set yourself as 'a user' for that number on the 'My Digits' page and away you go.
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!