Hi, my iPhone gets amazing signal strength in some areas and horrible signal in other areas. Whenever my friends are with me though, their regular phones get great signal, while mine doesn't - even though we are all using T-Mobile. I've tried putting the phone into airplane mode and then turning it off again, but it doesn't help. I also saw that Griffin made a case that increases signal strength, but it only works with AT&T because of the specific frequencies of each service provider. Is there some way I could increase the strength?