Interesting problem with this CenturyLink ADSL Modem/Router I am stuck with for a while. I'm setting up a new business first at home and then when the time is right I'll move to a real office (and good direct Internet access) in town. Running 3CX Debian for 3CX v15 (most current build) I have the Firewall and SIP ALG off I put in all the port forwarding rules, ran the checker and all passed. Everything worked fine (2017-3-3). Now a few days later it fails, but the modem still has the port rules intact. Now I have gone in and removed all the port forwarding rules and rebuilt them in the Modem/Router. Now again it works fine as of 2017-3-5. I'll report back if it holds or fails again in the future..