While in a call click the volume up button on the side of the phone
Have you used an [attached] headset, ear bud, etc? If so, try inserting/removing the jack a few times. Sometimes, the tiniest piece of trash can get inside the headset port - and the phone "thinks" there's a jack inserted, even after you've removed it. Amazingly enough, I've seen this issue at my workplace - twice it was solved with this simple procedure. One phone, however was defective and required warranty replacement.
Hope it helps