Haven't read whole thread so apologies if I have missed anything crucial.
Log into your Server Edition server with Manager. Go to System > Telephony > Maximum SIP Sessions.
If this value is 0, increase to fit the number of SIP sessions on the IP500v2.
Let me know if that fixes it.
ACSS (SME)
One of these days everything will work as it should, and then we'll all be out of a job!