Absolutely. There a button called "Switch Devices" which lets you use any of the device that you have registered with the DIGITS app.
In addition you can transfer the call to any other T-Mobile subscriber. So for example if you answer a call from your in-laws, you can transfer the call to your spouse directly to their phone - without having to do a 3 way conference call.
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!