If the automatic new handset registration does not work by putting the handset in the base cradle, a manual registration process is available.

This is for the Accessory Handset model CL80113 for use with the following Base units: CL81113, CL81213, CL81313, CL82113, CL82213, CL82263, CL82313, CL82363, CL82413, CL82463, CL83113, CL83213, CL83263, CL83313, CL83363, CL83413, CL83463.

Press and hold the Handset Locater button in the base cradle until the In Use light is steady red. Release the button. The base display will say Registering...
Press the # key on an unregistered handset to start the registration process. That is it.