You've provided very little information to go on.
Depending on the method that you receive "outside" calls, it could be the port forwarding setting in your router or firewall programme. It could be settings on your gateway. It could be a Codec issue.
The 3CX server log might provide some guidance, if it shows any errors during the establishment of a call.
Did this issue just begin, or is this a new install? What about placing outside calls, is there audio?