microphone during call not working iPhone 5

Hi all,

i have this iPhone 5 that when i make a call the microphone do not work very well, during face time video recording all works fine only on gsm calls it doesn't work properly they hear me really far away i've changed the charging dock where the microphone is is possible that is the microphone on the back of the phone near the camera that cancels the noice during a call?

let me know

thank you