Your call could not be completed because the call was routed through an intermediate network that does not service the far site. Contact your network administrator for assistance.
Most likely Cause: Firewall or Router access list blocking the H.323 messages required.
If there is a gatekeeper involved the unit may not be registered properly.
To alleviate the problem of IP dialing, the H.323 standard defines the use of a gatekeeper.
The gatekeeper is a system that connects to the Internet just like the client terminals. The IP address of the gatekeeper is configured into the client terminals and when the clients “power up”, they communicate with the gatekeeper and transfer certain information to the gatekeeper that describes the vct. 
 Topic: polycom pvx (2006-Nov-01) [Wainhouse Research Forums]
 Video Conferencing – An Introduction [Ezine]
 Interworking Signaling Enhancements for H.323 and SIP VoIP [CISCO]