About 6 weeks ago my apartment became an att dead zone

Calls used to work ok but no they will not come in or go out. I calle datt several times and they say there is nothing they can do. I have several friends with att and no one gets calls in my apartment, friends with sprint still do. One of the guys at radio shack said I should try to "force roam" my phone to see if I can make my phone connect through a t mobil tower. How do you force roam an iphone?
Any other help for dead zones? I am going to have to cancel after 5 years with att if I cannot get service in my apartment
Thanks