asterisk users - Apr 2006 - Polycom IP 501 buddy list: Got SIP response 500 "Internal Server Error"

I've just set up my dial plan to use hints for each of my extensions,
and I've set up a buddy watch list on my Polycom IP-501's, however, I
keep getting the following messages every 5 or so minutes:

    -- Incoming call: Got SIP response 500 "Internal Server Error"
back from 192.168.1.106
    -- Incoming call: Got SIP response 500 "Internal Server Error"
back from 192.168.1.100
    -- Incoming call: Got SIP response 500 "Internal Server Error"
back from 192.168.1.106
    -- Incoming call: Got SIP response 500 "Internal Server Error"
back from 192.168.1.100

can anyone tell me how to disable this error message, or otherwise how
to prevent it from happening?  Thanks,

Mike