My question would be - How much time have you spent in the new area?
On a monthly basis, I travel to check on my Mom. When I'm there, there is NO TMO coverage; however, AT&T picks up and works fine. TMO's coverage stops ~80 miles before I get there. I've only ever had one issue and that is when TMO's coverage ends, and I'm on a call, it will drop the call. I can wait about 10 seconds, and then I'm up on AT&T and I can resume my call and all if fine!
I looked at the TMO map, and it shows "weak" in that area.
I check AT&T's coverage in that area, and it shows pretty good coverage! Even 3G, if you're in the right spot (with the right phone, obviously...) If you have your phone set to Automatic Mobile Network, it will switch automatically - assuming TMO has an agreement with the local AT&T system.
I can relate to your issue. Where I live, there is GPRS TMO service only. AT&T has EDGE. Makes is slow going on surfing and any other data activities. I can, at times, "trick" my phone to switch over to AT&T's service so that I can use EDGE when tethering my phone to my computer.
Also, for me, what really sucks - where I work, my phone shows SOS most of the day as there is AT&T service but not TMO. Those that I work with on AT&T use their phones all day. I can use my during lunch....
Good luck!