Hope someone can help. Installed an IPK2 with a VMP-U40 VOIP card. Programmed card as Meggaco MG16. Using dTerm 32-button phones with IP-R adapters. Have switch in main office with 3 satelite sites. Using IP phones at the 3 sites. Phones at each site can call the main office and vise versa with no trouble. Phones at each site can call a phone at the same site with no problem. Phone at one site can call the DID line at another site or call the auto attendant and be transferred to another site with no problem. Trouble is when a phone at site 1 calls the three digit extention of a phone at another site phone says calling and the DSS button for both phones lite up. The phone at the other end rings and says Calling from The extention. When the phone answers both phones say talking but their is no audio on either phone. Only happens when one remote site calls another. I am totally stumped.

I had something vaguely similar on my IPS 2000. So here is what was my problem:
I had a managed switch on the IP network, that, after a lightning storm, went back to its factory default settings, and thus its IP address conflicted (became the same as) with the IP address of another router on the network. From there on, there was no audio path from my IP phones on the remote site, back to the switch.

After correcting the IP address, the problem was solved.

I understand you have a different system, and somewhat different manifestations, but it might be a good start to look at IP settings of different nodes on your network.
I hope this helps.