Then there are at least 3 possible explanations.
1 The "call" microphone has a "loose/bad contact. (send it for repair/replacement)
2 The phone "thinks" there is a earphone plugged in the 3.5 jack. (dirt can cause it) Send it for repair / replacement / Clean the jack good with for example a dry toothbrush)
3 A software problem. ( Possible solutions are : soft reset, hard reset, Lumia software recovery tool)