This reply that I gave to someone else, apparently solved their problem.
"When you say that you can't hear them, do you mean that you can't hear them speaking when they answer?
If so then either you have your call volume turned down, or you have a problem with your phone. Don't forget that the call volume on most phones is separate from the ringing volume (i.e. if you turn up the volume when the phone is not in use, you will be controlling the ringing volume - turning up the volume during a call will affect the call audio volume).
On later versions of Android, if you press the volume keys when the phone is not in use, an indicator of the volume will show at the top of the screen, with a bell symbol. If you tap it, it will open out to show your other volume settings, which you can drag left and right (lower and higher) or tap once, then and then use the volume buttons. "