Not quite sure I understand what you are saying here, so I'll say how it seems to work. Which may or may not be consistent with what you said.

I was roaming in Spain and France. I was in the beta. Let's say that my regular T-mobile number is (777) 777-7777.

I could install the DIGITS app on a phone using a different T-mobile SIM with a different number (not the one with all of the 7's.) I chose data connection in the DIGITS app, and it worked just fine on the throttled 256kbps roaming data. Free incoming and outgoing calls with the DIGITS app, all on my (777) 777-7777 number.

If I installed the DIGITS app on the (777) 777-7777 phone, and chose data connection - I could make outgoing calls on the app just fine (using the data connection), the called party would see the (777) 777-7777 caller ID.

Incoming calls were a different story. The incoming call would come in both through the DIGITS app and natively on the phone, and I could not answer the DIGITS call. I suspect that this might be somewhat hardware dependent based on the phone you have.

If you have an extra line, best to use that if you want to get incoming calls on your DIGITS app.

Another possibility is if you are travelling with someone else with a T-mobile phone - you install DIGITS on each phone linked to the other SIM - i.e., the SIM in the other phone. That way incoming calls will not ring on the cellular side and DIGITS side of the same phone at the same time.

If there was some way to turn off the cellular calls without turning off the cellular data - that would be ideal.