Used FON-870i-H cordless handsets will not register to base.

Hello,

I have a client that I am working on who has a FVC40 and a single registered FON-870i cordless handset. They would like to have a second one setup so we acquired three new FON-870i-h cordless handsets. The problem is that I cannot get them to register. They say deregistered so I followed the instructions I found (register, enter code, and reboot base), but it is not working. I also setup the extension with the base MAC address on the FVC40. Any suggestions or insights?