Hey guys,
I'd like to add to this post. My iPhone 4 is having pretty much the exact same problem, though with slight differences.
When I try to make a call, my speaker button is selected, and I hear nothing. I am actually placing a call, however, I hear nothing, and the person receiving the call hears nothing. Totally muted. If I press buttons on my keypad however, the other end can hear it, though they are slightly distorted.
Then, sometimes it works like nothing is wrong ( I can place calls etc, hear, etc.) And then, being on a call totally crashed my phone, and it went to the Apple screen. And THEN, later on, it crashed again on a call, and the screen went totally black, and the phone started vibrating! And it would not stop vibrating - I tried hard reset, I tried plugging it into iTunes...nothing. It had to sit there and vibrate (very annoying), until the battery ran out (all with a black screen). Then, I plugged it in after the battery died, and it gave me the "plug-in to charger" logo like everything was fine.
When I got the phone, it had 4.3.3 on it I believe, and had the same problem. Then I updated it to 5.1.1 desperately hoping that it would fix it, and it still has the same problem.
My microphone and speaker (hardware) work fine, I record voice memos, record video, watch youtube, send emails, even listen to voicemails! I do everything else the phone is intended for, except make and receive calls. I took it to Apple and they were clueless. Basically they said reset it if it's software, and I did (erase all content and settings) and no avail. I hope we get this figured out.