i have a jailbroken iphone 3gs for tmobile. well the past few days i have been losing signal, and then gaining it and having it show that i have signal, but i cant text or call anyone. also i have been having a lot of calls dropped, and when sometimes i just cant even call or text anyone, like right now. how can i fix this or why is this happening

FYI: i am not in a different place i am in the same town and home where it has worked perfectly, and i have turned it off and then back on, VERY MANY TIMES