Very interesting. This contradicts how Google advertises the service though. They say the transition is seamless. Unless they are playing fast and loose with the definition of seamless, this does not seem to qualify.

According to their website it is expecting signals to slowly get worse and have time to start transitioning before it drops out. A sudden loss of wifi for whatever reason probably confuses it. I would expect walking down the street away from your wifi would cause a smoother tansition.

I also expect those dialer codes are not meant for use during a call, and that is why a transition is queued up.

Have you tried driving from a Sprint deadzone, where you are on TMobile, to a TMobile deadzone where you would have to transition to Sprint? This would show if it can actually transition between the two during a call. Which others seem to say works.