I got the iphone4s once it came out and I've been having some issues with calling. 4 out of 10 times I dial a number, it doesn't ring. I have to either manually end the call or wait for it the phone to automatically end the call to redial. I can hear the phone ringing the the 2nd time i dial. What could be the source of the problem? I turn on wifi the whole time, could that be the problem?