Well that doesn't seem right does it? I know it has been a few days since you posted this but I wanted to check in with you to see if you are still having this issue? Did you end up calling our technical support team about this issue? We may need to open up a trouble ticket for this issue for you so please let me know where we are on this one.
Unfortunately, the problem persists. I have this problem since the beginning of DIGITS beta, and has not been solved.
I prepared a small video demonstrating exactly what happens. It is available at: T-Mobile: call quickly drops and reconnects - YouTube
The video is short, keep close attention, in particular to the upper left corner of the phone, wherein the call status is displayed ("calling", "call ended", etc.). I make a call to 611 to demonstrate the quick disconnection and reconnection of the call.
At 0:00, I start dialing 611.
At 0:09, I press the "call" button and the call. Pay close attention to the upper left corner of the phone; it will display "calling".
At 0:11, call is "ended" by cell system (or something). Upper left corner says "call ended" in red letters.
At 0:12, call is reconnected by cell system (or something). Upper left corner says "calling" again, and call proceeds normally.
When DIGITS is enabled, this "call ended"/"calling" switch causes the virtual line to be deselected, and the primary line to be selected upon reconnection of the call.
That is it.