Hey guys - I have the Otterbox Defender case also and thought this was the issue so I removed it. Still the same issue. Yesterday I called the Apple store, made me an appointment with the Genius... they REPLACED my phone - said it appeared to be a hardware issue (microphone?). I also had purchased the warranty thingy, so there was not a charge. So far, the new one is working fine. I did put it back in the Otterbox case - It is a fairly new case for me and I am still out to lunch about whether or not I like it - seems a little on the bulky side to me, but, hey if it saves the phone on a drop I'll be okay with it. Good luck...
Chiky