If you pick up a call and can't hear the other person, it's most likely that you accidentally muted the phone's earphone volume (turned it all the way down). You just need to turn it back up in two simple steps:
1) Make or receive a call on that phone.
2) During the conversation (when the call is connected), press the volume up button until the volume is at the desired level.
3) That's it! You can now hang up.
It's that simple. There's no need to do anything else. The volume level you set will be saved for the next call.