When Emily broke the screen on her 2yo 4S I paid for a refurb replacement so I'd get a fresh battery. I'd heard good things about Apple refurbs.
It now sometimes happens sometimes happens that her microphone doesn't work during calls. A power cycle or simply waiting a while can resolve it. Happens about once a week. I think this is the cause:
I'm sorry to tell but it is 99% sure that your 'audio compiler ic' is the issue. This is the IC which performs the noise cancellation. It is only active during calls and not during memo recording... Reballing this IC is 99% gonna solve your problem. However this requires special tools and good knowledge to be performed..."
There's a possible workaround using the hearing aid option, but I'll take the phone back to the Apple Store. I wonder if they'll believe me -- I'll try to catch it on video.
It looks like this is a known manufacturing flaw:
If this audio compiler doesn't work, then the person on the other end will not hear you - or they will hear a very muffled voice, or a lot of static. Due to what is largely believed to be a manufacturing error in Apple's Chinese factories, a number of iPhone 4 models were assembled with this tiny chip soldered to the mainboard (motherboard) improperly. Either too much solder or glue was used, and the connection is tenuous at best.
The problem is that the phone could work perfectly for a very long time, and then after a drop (even one that doesn't break the glass), a hard jar, or even for no reason at all, this chip loses its connection to the mainboard and causes the problem you may have experienced.
There's a 178 page Apple thread on this problem: iPhone 4S - Outgoing call no audio. Given the length of the thread it's disturbing Apple missed the problem on our refurb. They should offer a recall.
Update: I found if I gently tapped the 4S on a surface I could trigger the microphone loss, so I was able to get a recording of the drop out.
Update 9/7/2013: Apple service-swapped the service-swap (so now I'm two phones removed from the phone AT&T would have unlocked in 2 months, but I'm told AT&T can manage this if I have the paper trail).
It wasn't easy though. I'd power-cycled the phone to switch SIMs, and last time I did that it took days before I could replicate the bug. So the tech couldn't replicate the problem -- and nobody at the Rosedale Minnesota Apple store had ever heard of the (alleged) audio compiler microphone cut-off defect. It took my poorly done video of the defect in action, a Google search with precise hits, and showing this blog post to get the exchange.
I sympathize with Apple here -- a hardware exchange for a non-replicable defect is a lot of money to lose. I wonder if they're not allowed to use Google though.