The microphone only works when using speakerphone .

I have a samsung epic 4g. Yesterday the ribbon cable for the touchscreen decided to break. It just so happens that I had an extra phone as a result me getting even with my phone insurance company (another story). Rather than just activating the other phone I wanted to device to remain configured the same as it was, so I opened them up and swapped the mother board etc. This appeared to work but when making a call I noticed no could hear me. I could hear people, but the same was not true. I tried putting what I assumed to be the microphone ( the piece at the bottom with the speaker that the white antenna cable connects to) from the old phone into the new phone housing containing the guts from the old phone. Still no functioning microphone (I tried using a voice command on the phone just to check it wasnt something to do with calling). I did notice however that when the phone is on speakerphone everything functions normally. Help please. I thought I had this figured out, but it appears I do not.