Hi!
A couple of questions..
- Is there a firewall between them?
- Are they on the same subnet?
- Did you specify the right subnet mask?
- Did you specify an IP to connect to the PBX or an host name? If you specified an host name is it resolvable by DNS and did you provide the phone with the right DNS server?
You could try activating sip debugging (sip set debug on) but I would try checking for things like this with the message it's giving you...
Good luck and have a nice day!
Nick