I had similar problem (but with Tandberg) until I added to my application somewhere in my FormCapabilitySet() line
endPoint.SetCapability(0, 1, new H323ControlExtendedVideoCapability());
This will add H239ControlCapability to local capability list, so remote side will understand that your endpoint can work with h239. I don't know maybe this was fixed in h323plus.
But with Mirial I manage to open only from Mirial to my application as the other way I'm getting "ERROR Open Channel. No Remote Support". I will check that.