I was having this exact same issue on my 8.5(1) cluster with 9971 phones (that did not have video cameras on them) and CUCM SIP trunks to a third party provider.
I was able to get the phones working by selecting "retry video calls as audio" on the SIP trunk. As the previous poster mentioned, the phones are video by default, so they try and use this capability by default.
I'm not able to access my old voice mail messages all of a sudden. The recording says something like 'the message is currently not available'. This has never happened before in all the years I have been using this system. I have t...