No, OPUS does not appear to work.
Set up extension 1000 in UCM using a GXV3275 and both sides (UCM and phone) were set only with OPUS.
Called from extension to VM on UCM, phone connected, no audio and then a bye from UCM. correct.
. Note that the BYE comes within 2 second of the ACK from the UCM. VM is enabled on the extension and the default greeting is longer than 6 seconds. I suspect that while I could not hear the audio, it was the result of the GSM recording method not being able to be transcoded into OPUS and hence the BYE from the UCM.
I then dialed in from a SIP Trunk, which is using G711u and G729. The phone and extension were left untouched with regard to OPUS being the only codec. Extension 1000 was the destination for the inbound route. The call was received by the UCM, but there was no attempt by the UCM to send the call to the phone. The UCM responded with a 488 which then caused the 2nd failover trunk to try which resulted in the same thing.
I then added a g711u codec to the extension and the 3275 and placed them as the 1st codec in the priority listing.
I repeated the same tests to call VM and using an inbound call from a SIP Trunk and both calls were made without issue.
I then took the same GXV3275 and configured it as an extension on a different make of PBX that also purports to support OPUS. Again, OPUS was set as the only available codec. I had no issues in using OPUS as it worked fine with the other make. You will note the BYE is from the phone when I hung-up, not from the PBX as was the case with the UCM.