I am having a similar problem, but intermittent. Sometimes the calls all work, yet while talking for a minute or so, sometimes they suddenly can't hear me (no, its not the mute button - its in correct position). Strange thing is that they can still hear the tones from the key pad if I press them.
Probably 3 calls out of 5, it is either I can't hear them or they can't hear me.
I think this is an AT&T profile / configuration problem and is not a iPhone problem. Does anyone have this problem on Verison or Sprint?
I tried with/without siri on. It started when I added the new iPhone 4s to my ATT family account, and it affects my other iPhones on that account, including a iPhone3. Apple worked with me to solve the problem, and I have tried 3 new phones - all have the same intermittent problem. Sometimes in my calls they can hear me but me not them; or, I hear them but they don't hear me. Does not matter if I am calling or answering.
I called ATT and they agreed that it was probably not a phone problem but a "profile" problem. It took 45 minutes for them to confirm this and escalate to 2 level support... Now they tell me it will be another 5 days before they can fix the problem.... Time to switch!?!