On the back of the S12 Base unit there are 2 ports. One says "Handset" and one says "telephone".
Unplug the "Handset" from the base of your existing telephone and plug it into the S12 base unit where it has the "Handset" icon.
Then take the short cable that came with the S12 and plug it into the port on the base of the S12 where it has the "telephone" icon, Then plug the other end into where your "Handset" was on your existing telephone.
Plug the headset into the port with the icon of a "headset" on it.
Route the headset cord through the cable management channel on the bottom of the amplifier.
Make sure the AC Power is plugged into the wall and the configuration setting should be on one where the call is clear to you and your caller.