Have a client with a Coral IPx500 system. They have a remote users with an IP flexset that is unable to make outgoing calls. Sometimes he can't break dial tone (even on an internal call). Sometimes he starts to dial and gets a reorder tone and "ILLEGAL" on the set's display. Incoming calls ring, but only get incoming audio. I recently troubleshot some firewall issues and had it working perfectly, and unless someone isn't telling me, there have been no changes made.

COS is set the same as everyone else's. Set does not have a passcode, and is therefore not locked. Kind of stumped.

Sounds like UDP blocking. I would DMZ that phone and try it that way. Also call another IP phone to verify the phone actually works. Could also be a bad phone or handset cord/switchhook.