I am using UCM 6.1.2, I face a very peculiar problem wherein once in every week my conference and transfer does not for all the IP Phones, and once I restart the IP Voice Media Streaming Service, it starts working fine. I have created the 2 Media groups, one for hardware resources and the other one for Software resources.
We had a similar issue. The installer set up the VG as h.323 and set to require MTP resources for every call. Some would not be released. Eventually, we would run out of MTP resources, and we'd lose conf, trans and new calls.
The solution was to go back to MGCP. That was about 3 months ago and no issues since.
Good luck and HTH