Can you elaborate any?
Air Gaps are usually a way of securing and isolating systems possibly to the point of where they connect to no other devices and have security measures to prevent snooping. Obviously, there can be connections provided that the communications path is also similarly secure and immune to hacking/snooping.
You might need to take a phone and then test next to the 3CX system to see if that works while using the 3CX softphone set to CTI mode. I assume that some phones may not support and/or may need to be upgraded in firmware. You then use the softphone to dial out which should then communicate to the desk phone to implement the dial. You can wireshark it during the test to see how it works and then perhaps you can take the observations and apply them to your network and/or whatever CTI aspect you have in mind.