Thehas another bug, this time outbound calls have no in coming audio and its across all three carriers. The problem seems to revolve around some sort driver issue between the iOS software and the 4S hardware. People have no issue with incoming calls or other aspects of iOS that use the earpiece, such as Siri.
According the the 29 page thread developing on the Apple forums the way everyone seems to be getting around it is to send a text message asking the person to call or calling until the call goes through with out the mute button on.
Users say that the problem remedies itself when speakerphone is toggled on and off, Siri is activated, or headphones are plugged in, then unplugged. But obviously, these are just workarounds for a problem, not a fix.