Ditto what possumal said but...
First turn on the main unit, then turn on the remote (hold button for a few seconds, then it will light up). You can choose the call that you want to use on the remote, then press the send button to send it over to the main unit. Then press the mute button to start the sound on the caller. Press it again to stop it. (this is from memory, but I think I got it right)