Is it just the ringback tone you are not hearing (after the first one)? Is the mobile App able to answer the call? Is there two way voice? Does this happen only when calling a 3CX App? Only on that one phone, or to others? From one caller, or other originators?
We see this happen a lot when a cell phone is in the loop, but is out of coverage. The call is instantly picked up by the cell and goes to VM - hence one ring and the call is "gone". If this is the problem, you can use Rebound.