If you answer a call and can't hear the other person, it's most likely that you've accidentally muted the phone's earphone volume (turned it all the way down), and you just need to turn it back up in two simple steps:
1) Make a call from that phone (or receive a call).
2) During the conversation (when the call is connected), press the volume up button until the volume is adjusted to your preferred level.
3) Done! You can now hang up.
Yes, it's that simple. You don't need to do anything else. The level you set will be saved for your next call.