Hi, The handsets supplied with your system would have come pre-registered but if you adding an additional one you will need to refer to the registering additional handsets section of the user guide (page 81).
The best way is to get a copy of the manual is online from BT.
The following link should do the trick.
http://www.shop.bt.com/content/uni2/documentation/498l/025455.pdf
Jon
November 2007