iPhone 4S with AT&T misses calls

Hi, I've been having this problem for a couple of weeks now....

My iPhone 4S has been missing calls. When someone finally reaches me they say that they have tried to call several times but it just rings and goes to voicemail; meanwhile I have had no missed calls.

I have tried turning airplane mode on and off, turning the phone off, resetting the phone, restoring the phone, restoring to a new phone and it still seems to miss calls.

I have between 3-5 bars with 4G on AT&T. Is there something else I can try?
Thanks---