Try using the app "open signals map" it should show the towers around you and they're signal strength. Also if you go into the dailer, hit *18 then call. Give it a second or two and you'll hear 3 beeps a pause, then 3 more beeps. This makes the phone find the strongest tower/signal and lock the device to it until you leave the area and the phone needs another tower to continue giving service.
Hope it helps