I've made a change to the network with access to WAN and adjusted services accordingly. However, with 3CX (Windows installation, and then replaced with a Debian installation) I'm having such a difficult time getting 3CX working again. I'm receiving the following log messages when registering SIP trunks on 3CX (note: I've altered my providers details). Registration at T-123456 has failed. Destination (sip:firstname.lastname@example.org:5060) is not reachable, DNS error resolving FQDN, or service is not available. 05/09/2018 11:19:04 AM - [CM504005]: Registration failed for: Lc:10004(@T-123456[<sip:email@example.com:5060>]); Cause: Cause: 408 Request Timeout/REGISTER from local Fact 1: I'm able to register SIP trunks using a Yealink phone system without any modification to the setup. Fact 2: My hardware firewall is intercepting some packets from 3CX, flagging them as a Land Attack as the source and destination addresses are the same (of my 3CX IP server). Fact 3: DNS is working on the 3CX server. Fact 4: In diagnosing this issue, I changed the 3CX IP and the SIP trunks registered immediately after. Fact 5: The router and firewall are correctly configured in respect to port forwarding etc. and there aren't any issues with access to the WAN. Fact 6: I've tried other SIP trunk templates without any success. Fact 7: The 3CX firewall test fails but then 3CX was working perfectly before when SIP trunks were registering. There isn't any doubt this is a 3CX issue when looking at all the facts. Are there any suggestions?