People can not hear me while calling

Does this happen all the time, or only when the speakerphone is being used?

Others have had similar problems - they can hear, but others can't hear them when they are in speakerphone mode. It's a problem with the ambient-noise reduction microphone (the tiny hole near the headphone jack). If the microphone works for recording in other apps, but not when you're talking on the speakerphone, bring it to an Apple Store, they'll swap it out for you.

I'm having the same problem. I've rebooted, recovered, reinstalled and uninstalled apps, tried it without any installed apps, tried with no apps in the background, tried to mute/unmute the call, tried with and without speakphone and I still have the same problems:
some time people will hear me fine, sometimes in the middle of the call they stop hearing me and sometimes they can't hear me at all.
what do?