Why is my iPhone on mute every time I place a call?

Stepped on the front of the iphone, cracked screen/ digitizer. Phone still received calls and could hear callers perfectly, though they could not hear me at all. I replaced both screen and digitizer and every function on the phone works perfectly with the exception of the lingering problem of callers not being able to hear me. Now that I can see my screen it appears that when I either make or receive a call my phone is muting the call from the instant that the call connects. The mute button on the screen is always pressed (has the line through it). If i make a call and hold the phone away from my face the prox. sensor seems to be working OK as the screen stays lit, if i move the phone to my face screen goes dark. If I touch the mute button in an attempt to disengage mute the screen goes black. I know its not software as I reset everything updated to 4.2.1 and no luck, went back to 3.13, no luck. Back to 4.2.1 and no luck. Any help would be greatly appreciated. I have searched for days on every board and forum I can find and have found nothing. Thanks in advance.