After a planned maintenance session and reboot, we started experiencing failed SIP trunk registrations. 3 different trunks (different providers) all failed, each reporting DNS/network issues. However, pinging the registration servers by hostname from the 3CX machine produced a correct IP resolution and ping response. Only by using the IP addresses of the SIP carriers' servers were we able to successfully register a trunk. Not ideal, but functional. I have not found any unfamiliar or incorrect IP addresses in Settings>Advanced>Customer Parameters (another user reported this issue and attributed it to an old, incorrect local IP address buried within one or more of the parameter fields) DNS is working fine both on the 3CX box and elsewhere throughout the network. I have tried using public DNS servers on the 3CX box with no success. I'm out of ideas now. All suggestions appreciated.