In all seriousness (my humor stinks), I would check reception. If it's THEM hearing YOU and you suspect a hardware issue, we can try to break the process down into what it uses and then try to test the variables. We know it uses a microphone. In fact, I think it uses 3 of them. You should be able to easily test this by doing something else that requires a microphone (like creating a voice memo) and see if, when you play it back, it's muffled.
If you suspect some other piece of hardware that is supposed to control either the compression or decompression of the sound (voice), maybe you can try something like facetime with a different phone. I really don't know though... I would guess that utilizes different hardware/protocols to transmit voice and picture... but hey, it's worth a shot in troubleshooting just as another bullet point on your "tried" list.
You can kinda see where I'm going... I would recommend taking this approach for troubleshooting hardware. It's really, really tough though cause there are so many variables that are out of your (our) control that you won't be able to test. I'd hate for you to go get a new phone just to find out that your carrier was having a problem in your area or something