Iphone 4 here mic works for everything but voice call i can use different types of apps to call with and it works but when using the regular gsm network calls doesn't work that not and a software or program would not cause it to hang up and not work. If it picks up your voice on them it should the gsm calls to.it hardware issue not software issue.
Interesting now that I have this issue myself. For me, seems to co-incide with the lastest upgrade (but others around me have the same model and upgrade without issue).
Symptoms I have:
Microphone does not work for telephone calls.
Apple headset does
Speaker does (isn't it the same mic?)
All internet/facetime/camera application all have microphone working.
Also try blowing or tapping on the mic - this can be heard by the receiver of the call.
Also if you put your mouth right up to the mic (ie over the bottom of the phone) a 'garbled but understand dialogue can be heard, but move the phone away from the mouth at any distance, dialogue at the receiver discontinues.
It looks like the reset is the answer, but I haven't done one yet, until I've eliminated all possibilities.
Anyone identified a setting or cause (and therefore solution) as yet?