We don't really have a choice as the whole reason for switching to 3CX was to save money for our church. It has, so far. But if we were to switch to some other provider besides Azure with their sponsorship, we'd end up paying more than we were before we had 3cx, so it would make no sense. Further, because we have Microsoft365 with a non-profit premium license, it again makes more sense to stay with Azure for integration. Further still, I am well versed in Azure, so if I knew what I needed to do to get the upgrade to work, I would/could do it.
I was even able to size up our VM to Standard B2s (2 cpu's, 4GB ram) with 30GB SSD so all recommendations match or exceed, and still stay within our sponsorship.
When I look at the error log, I see a single error concerning a package list file (microsoft-prod.list) which is automatically added when the Debian VM is created (I played no part in that file's existence). Initially, when I removed this file (per the error log's instructions), prior to recreating the VM, apt stopped working so no upgrades/updates could be done. I manually reinstalled apt and its dependencies, and the upgrade made it much further, but still failed.
The error log I receive is attached. This is from two days ago, the last time I attempted the upgrade. This attempt was done AFTER a complete deletion of the VM, a fresh new VM using the 3CX server setup from Azure Marketplace, following the instructions at
https://www.3cx.com/docs/pbx-azure-marketplace/ to the letter, updating 3CX to 18.0 (Build 20), and then running the upgrade. The server is in un-edited, off the shelf state right now. Everything is working fine, save doing the upgrade.
Trying the upgrade again. This time, after backing up, I moved the microsoft-prod.list file out of the packages folder. I did this the first time too, but there were other problems with the Debian install (that apt was missing, for one) which made it fail. So far, it's been about 8 minutes and no emails showing failure. Maybe a good sign?