It could be that prime line for that set is set for I/C when you go off hook. If you go into config programming. Feature **config password config, Show Configuration and go next to line access. Show Line Access and enter DN. Hit next until you get to Prime Line and then change to Pool A (most likely, but check another ext. first) If it comes up Terminals and Sets, show it and enter the ext. Show that DN, and then show Line Access, and then go next to Prime Line....
If this doesn`t work, give a little more detail as to what type of set, what appears on other sets when you go offhook, is there a line out button on the other phones, (button inquire it if it does) and more importantly, whether you see Configuration or Terminals and Sets when you go into Configuration programming. It makes it a lot easier if people know what programming version you have.
Schlum