I had a similar issue that was coming and going I struggled with for a long time. I don't know that this would be a fix, but I will share what we found.
We had Verizon SIP trunks come into 2 different CUBEs at 2 different geographic locations. For the most part, we had from years prior, all calls coming in and going to g729 for most sites. We did have calls going to the region our UCCX servers were on set to go to g711 which is what UCCX requires, but some of our scripts hold music were on g729 and connections from UCCX to the agents partitions at g711.
It wound up being really tricky as only every so often, for about a day or 2, we would suddenly have this issue and after months of not being able to hit this with TAC at the same time of it happening, we finally saw with TAC, them saying that Verizon was not answering the handshake on codec, and then go to Verizon on that and them saying they are not getting a handshake. Nobody really had an answer other than that as the codec changes, someone was not responding with that handshake as we went to MOH (g729) back to UCCX prompts (g711) and back to MOH (g729) and then to an agent (g711) so we had no audio and calls drop.
Finally; I went through and flattened them all to be g711 all the way through. I edited every script to not access our MOH on g7.29 which I learned g729 was not meant for music anyway and used ones we had within g711 or via built in prompts. No more handshakes from flipping codecs back and forth. Was a huge relief.
