So I have ran into a bizarre issue I was hoping the community could help with. I came in this morning to our SIP lines down.... Needless to say it wasn't a great morning! So it looks like the SIP lines kept giving the error: Call or Registration to $$$$$$$$ has failed. sip:$$$$$$$$$$@inbound5.vitelity.net:5060 replied: 503 DNS Timeout; internal Or should I say, most of the SIP lines. We actually had 1 that would register fine. This would lead one to investigate DNS, so that is what I did. The odd part was that I could resolve everything just fine. I even switched the server to a public DNS server and could still resolve fine. So a little background... We have a Mikrotik router as our gateway device. We are in the process of switching everything over to a Watchguard. About two weeks ago I moved the phone server, it's public IP, and all NAT rules over. The public IP never changed in this process. I ran a firewall check then (and today) and everything necessary passed. I say everything necessary because we only allow about 200 of the media ports through, but that wouldn't cause the issues. So when this occurred this morning I actually moved the phone server back to the Mikrotik and it still failed to register. I even created firewall rules on the WatchGuard that allowed everything in/out. Still no luck. The last thing I did was go into the settings and change the network settings to use STUN instead of a static IP address. I flipped back to the SIP lines and Boom! all registered. Am I missing something? Does this make sense? Can someone help enlighten me on what I am missing?