I'll add onto the dog pile...
Sprint // 16gb 4s black // mic intermittently doesn't work.
Its pretty much been an ongoing problem since getting the phone so I've attempted to establish a pattern.
The mic seems disconnected. I can hear callers, they cannot hear me. This happens on both incoming and outgoing calls. To isolate if it were an 'in call' issue I immediately tried using siri after these incidents. Even yelling directly into the mic the feedback light on siri didn't move illustrating that the phone is recieving ZERO input from the mic.
I've eliminated the possability of it being a hardware issue as I can simply reset the phone with a power cycle and it works flawlessly.
I began taking note of the circumstances before the phones issues each time and a pattern arose. The mic would stop working only after my phone has synced up with my in car bluetooth (GM Factory System/2009 Avalanche) and would continue to have the issue until power cycled.
It appears for some reason that switching into BT mode in the car somehow gets the phone off track.. it will work completely flawlessly with a headset, the in car BT, or even a wired set of ear buds and mic, but as soon as I'm required to rely on the phones built in mic I have to reset. BT seems to be the trigger in my case. I shut it off and unpaired my phone from the in car system and had zero issues for 2 days, just didn't use any bluetooth. Re-paired the phone and truck days later, made a few calls, and then when I exited the vehicle... back to not being able to make calls again! The phone simply cannot switch from relying on an auxilary mic back to its internal mic for some reason.
Took to apple store and they were able to recreate the problem in store, but the 'genius' was confident that a new phone wouldn't fix the issue as he admitted to replacing a couple dozen phones already and most of those came back still having an issue. I may just return the phone to sprint and go back to an android device. I love apple products, on an imac right now, carry an ipad and macbook everywhere, backup to a time capsule, have appletv, love the form and function of the 4s... but if I have to power down and reset the phone every time I get out of my truck its no good to me.
Never had this problem with my evo 4g or evo 3d devices... guess if the fine folks at apple don't care enough to fix a glaring flaw in their product I can just go back to android. Unfortunate.