If it is a single set, behind a router, then STUN
should be enabled on the set. If you look at the 3CX logs, just after the set has been powered up, you should see a registration that includes the public IP (of the router) and the port you assigned the set (probably 5060). If the set you are using does not have STUN, then have a look at this thread and in particular, the reply from Sy
no-incoming-calls-19487.html#p103592Not knowing what else is on that network, there is a possibility that another device is causing a conflict/confusion with the ports on the router.
The easiest way to confirm a router issue is to move the set to another network (if possible), this might save you some time by eliminating the router as the cause of the problem.
There is also an option in 3CX, for each extension that you might try changing, it might make a difference...
PBX delivers audio, it used to be called something like that, the name may have changed slightly in recent versions.