I am having intermittant issues of no audio, or hearing buzzing when using the Opus codec. A simple test is to sip:888@conference.freeswitch.org - a freeswitch conference bridge which supports Opus. I have also tested calling my GXV3275 - it seems that if the DP720 makes the call, audio doesnt work, but if it receives the call it does work.
If i remove the Opus codec, and only do G722, then audio works fine in all cases.