This is a symptom of the problem you solved with the MWI subscription type.
This, on the other hand, looks like a TCP failure. The most likely cause of that (that I can think of) would be the firewall resetting the TCP connections, a time-out somewhere, or the phones restarting the connections and the server trying to pick up in the middle of a conversation. All three of those could be caused by a bad switch module or a stateful firewall/switch losing its mind and forgetting who your phones are. Of course, as with most things IP, it could also be squirrels or a coincidence.
The more we talk, the more it sounds like bad infrastructure. I could be completely all wet on that, but I’d really like to suggest starting from the network core and working your way out.