Perhaps this will be of help to someone.
I had a heck of a time pairing my Droid via bluetooth to a USED Panasonic Link-to-cell KX-TH1211B system. It was somewhat tricky.
Read the entire instructions - not just the "registering a cellular phone" section. Read the "Link to cell settings". Ensure that any previous cell phones (there were 2 in my case since the system was used) are deregistered and that the system is in discoverable mode. It might be straight-forward from there.