Hi, I have the same issue as well.
My iPhone 4 is on iOS5.01. Has been working perfectly for the past 16 months.
Few days ago, my mic started to give me problems. Although I had no trouble with FaceTime, Skype, Viber etc.
But when it comes to Phone Calls, using speaker mode won't work, using the original headset wouldn't work as well.
Hence my iPhone cannot make or receive calls as the other party cannot receive what I am saying.
I tried the voice memo app and when I speak to the mic (both top and bottom of the phone), the pin moves. This clearly shows that the mics are working.
I tried restarting my phone. Restoring my phone but it just wouldn't work.
I can't drop it by an Apple Store has I do not live in a country that has an official Apple Store. But there are authrorized sellers in my country. Since this is such a common failure, and I am out of warranty, can the authorized sellers replace one for me if it is deemed to be a hardware failure?
I am hoping that this is a software issue. So can the kind people of Apple attend to this matter? I think I speak for everyone here when I say we would very much like your response on this issue.
So please let us know what can be done or else a bunch of us will have a phone that cannot make calls.
Thank you.