The base unit and the remote unit come already paired with each other in the factory by a process called subscription. If you replace your remote unit, or if your remote unit loses its subscription (this is shown by a rapidly flashing On-Off indicator), you can perform the subscription process as follows.
Hold down both of the talk volume controls located on the top of the base unit. The base power indicator should begin blinking.
Hold the remote unit’s listen volume control in either the up or down position.
The remote unit’s On-Off indicator will turn on (no flashing).
The remote unit will subscribe (or re-subscribe) to the base unit, after which the base power indicator will go from blinking to solid, and the remote unit’s On-Off indicator will turn off.
Unplug the AC adapter from the base unit and then plug it back in.
Press the remote unit’s On-Off button to allow the link. You will now be able to use the CA12CD.