Well, first thing: can you connect to the 3CX server from inside the network? I assume that you can and all works as intended?
3CX indicates the following ports are used:
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The mandatory ones to be forwarded are, depending on what you are connecting (see notes in the table):
- TCP 5001 or 443
- TCP/UDP 5060
- TCP 5061
- TCP/UPD 5090
- UDP 9000-10999
You can configure these on your MX:
Security & SD-WAN -> Configure -> Firewall -> Forwarding rules