I have the feeling that the issue has not yed been positively identified by Cisco in all the ramifications.
E.g. I have it for calls coming in via ISDB BRI w/ 12.4(11)XJ4, no matter the phone type or release. But calls from IP phone to IP phone, are OK.
If you want to try with a different FW, rather than going backward, I suggest 8.0(7) that is the latest for 7940/60.
Hope this helps, please rate post if it does!
As mentioned above, CSCsj18014 was indicated as the applicable one, but reading the description, it doesn't seems so to me. Another bug that appears in the same page, is reported without ID and I couldn't pull from search.
I guess your only choice is to open your own TAC case an see what you are told about. You should ask immediate escalation to CME team to avoid loosing time being handled by unqualified CSE.