This works I just tried it on a new handset for my old wharfedale engage set.
TO REGISTER A HANDSET
[At the base]
1. Press and hold the the PAGE button on the base station for at least 3 seconds. You have 90 seconds in which to register a handset.
[At the handset]
2. Press MENU and scroll to ADVANCED SET and press MENU
3. Scroll to REGISTRATION and press MENU
4. If necessary scroll to display REGIST.BASE and press MENU
5. BASE 1 2 3 4 is displayed and the registered base number is flashing.
6. Enter the number of the base you wish to register to your handset. If you have only 1 base station, press 1. If you have 2 base stations, press 1 or 2, etc.
7. When the display shows PIN , enter the base pin 0000.
8. When the handset is registered to the base it is assigned a handset number.
Matt, April 2013