Can't you just go to the phone and enter 355-8-(number as you would dial it)? That would be typical way, assuming default feature access codes and 8 is the outside access code. If it gives an error doing that, make sure you have "Manual forward to public network" flag set to YES on the phone in question. Once you are able to set it with the feature code and the phone says "FWD ALL CALLS TO xxx-xxxx" in the display, try calling the extension internally first and see if it forwards through. If that works, call the station's DID number from an outside phone (like your cell) and report back.