I've spent about six hours now experimenting with the V66 Pro and didn't have much trouble making it work with 3CX. I was also able to get around the function key limitations. I've tested thoroughly and it looks like everything is working as expected.
On 3CX, I generated a configuration using the V65 template and sent that to a factory-defaulted V66 Pro. Afterwards, I logged into the phone's web UI and downloaded the phone's configuration. The configuration is a single XML file - which is what the 3CX template generates to provision the phone.
The 3CX template for the V65 does not store the function key definitions in the correct place for the V66 Pro, which is why it's limited to nine keys when using the V65 template. So I manually edited the function keys section of the configuration and moved the definitions where they need to go for the V66 Pro. It is a very simple change. The function keys are working as they should and I learned a lot in the process. I am only using 29 function keys for this test, but it's possible to define all 127.
There does not appear to be any easy way to store the edited configuration in 3CX, so there is also no way to send the edited configuration to the phone using 3CX. But the phone's web UI makes it very easy to import the file directly to the phone. And the file is saved to non-volatile storage and so survives phone restarts. Since 3CX can't send subsequent configurations to the phone anyway, there's little or no chance of 3CX overwriting the custom config file saved to the phone.
Again, the only changes I made were to the function key definitions section of the configuration file. Everything else remains "3CX stock" generated directly from the 3CX V65 template.
