I have an IP phone the end user is trying to set up. The extension is 744. The extension programmed in the phone is 744 and the IP address matches the one in the CIX. They are getting a server not found error. They have all the correct ports open on their firewall. What am I missing?

Set the phone to a static IP, maybe having issues with the DHCP server.

Do you mean "IPU not found" on the display? Can they ping the IPU address?