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The following ports must be opened/forwarded on the Firewall/NAT:
443 (TCP)
4887 (TCP)
6000-9999 (UDP)5060, 5061, 5062 (UDP/TCP, required to connect
Additionally, one of these to register SIP endpoints via Ringotel server using Terminal password:
5060, 5061, 5062 (UDP/TCP)