The issue in question is when the iPhone 4S (and according to some reports, the iPhone 4, too) mutes all audio on the caller’s end when placing an outgoing voice call. The person on the other end of the line can hear everything just fine, but on the caller’s end, there’s no ringing sound and no audio from a recipient speaking.
The issue has pushed the Apple Support Communities discussion thread past 110 pages with users expressing their frustration and calling for Apple to do something about it. Two of our editors, Rene and Georgia, have both had their iPhone 4 devices swapped out by Apple when the earpiece stopped providing audio (only the speaker phone would work).
It has been speculated that iOS 5.1 won't address this issues -- assuming it is addressable , but we'll have to wait and see.