If you are replacing a handset, you need to de-register the old one so the account can be assigned to the new unit.

On the handset go to settings.

Using the arrow keys, Scroll to Settings, press OK

Using the arrow keys, Scroll down to Registration press OK

Using the arrow keys, Select De-reg Handset and when highlighted click ok

If asked, enter the system PIN this is 0000 (4 Zero's)

Press Done.