I am answering the question posed about call hand off when using a third party VOIP solution on an Android phone to TMobile. While you can can do third party VOIP with SipDroid on Android, these calls are not tied to your TMobile account. You won't get messages and your calls won't go through your Tmobile account. It is essentially a different phone number.

It is possible with a lot of tweaking to come up with a system that will automatically poll your VOIP connection and transfer it to your mobille phone when you lose the connection. The hand off won't be perfect. The best thing you could do would be come up with a system that allowed you to manually transfer the call when you were walking out of the building. If you use Google Voice, this feature is included for free. Just push * and you can transfer a call.

UMA is clearly your best solution. You won't have to transfer calls and you probably won't drop them. Since your son is traveling between a few buildings on campus, he will quickly learn the cellular drop zones and will know adjust a little to deal with them. There is a tunnel near my house where I always warn people that I will probably drop the call and will call them back if I drop the call.