So I have an Iphone 3GS which, when I have no 3G signal shows "No Service", however I do have adequate normal signal. Basically, the phone isn't switching back to normal signal when I have no 3G. If I turn 3G off in the settings then the normal signal bars jump up to 4 or 5 bars. I have tested another Iphone with the same sim, and also tested the antenna connector, and these are both fine. I have also completely reset the phone, tried turning airplane mode on and off, and wiping the network settings. Does anyone know any reason at all why the phone would not automatically switch back to normal signal when there is no 3G? Doesn't seem like a hardware fault to me, as the phone works perfectly other than this irritating problem.
 
Any suggestions for me to just try would be much appreciated. As I said before, the sim and network are fine, so don't worry about suggesting that I contact my service prodiver  .
 
Oh by the way, the phone is running 4.3.3, as is the other phone which the sim worked fine in.