xd5100?

According to the manual (you DID look at yours, didn't you?
If you don't have one - you DID search fo rone, didn't you?

Anyway, the manual says:

Register a new BT Freelance XD 5100 handset to your BT
Freelance XD 5100 base
1. At the base: Press and hold for 6 seconds until you hear two beeps. You now have 90 seconds to register the handset.
2. At the handset: press, scroll to REGISTER and press .
3. Scroll to REG BASE and press .
4. Enter the base number (1–4). The numbers flashing are already in use.

Manual here: bt.custhelp.com/app/answers/detail/a_id/10850

Funnily enough, that was the first result I got when I searched for "XD5100 phone"

Derrick
January 2015