This has been found to be a common issue ( http://discussions.apple.com/thread.jspa?threadID=2475592&start=0&tstart=0 )
The problem is when on a speakercall to someone they cannot hear your voice, a speaker call is also a facetime call as audio is dealt with the same way as a speakerphone call. This issue is also found while recording video, occasionally the mic will work and record audio but most times it is just static that is recorded. This is not an issue with every iPhone 4 but is pretty widespread by the amount of people posting on the Apple Discussion forums. Please post in this thread if you have heard or any fixes or even if you have the same bug.
The problem is when on a speakercall to someone they cannot hear your voice, a speaker call is also a facetime call as audio is dealt with the same way as a speakerphone call. This issue is also found while recording video, occasionally the mic will work and record audio but most times it is just static that is recorded. This is not an issue with every iPhone 4 but is pretty widespread by the amount of people posting on the Apple Discussion forums. Please post in this thread if you have heard or any fixes or even if you have the same bug.