I have a SIP trunk true WAN between cucm9 and cucm10.5 and I have some problems when I want to make a conference between phones. When i place a call from cucm10.5 ( 7841 devices) and the person from cucm9 ( 7841 device) answer and I push the conference button and I place a second call to another nr. the first person is put on hold, after the second person answer i can talk with him but when i push the conference button the calls drops. After I call the second person I have the option to pus the conf button or swap button i can switch between the first user and the to the second user but when push the conf button the calls drops again.
Seems like a media resource issue. Could you confirm whether the phone on which you are pressing the conference key has conference bridge in the MRG/MRGL or in the DP MRGL. If yes, then also focus on the codecs negotiated between all the devices involved (IP Phone 1 --> Conf bridge,Conf bridge --> IP phone 2).
I change in both location to Media Resource Group List TO non but the same problem. First in 1 location was MRGL and the second cucm9.6 was NONE.