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.