1) In maint commands enter "locate features <ext> to see if the set is station forwarded. If it is then it means the user forwarded at the set.
2) Where the phone routes to, is it ringing the prime line? (bottom right key)? If not, then edit the keys of the phone it is ringing on. You may find that there is a key appearance of the 1st phone somewhere on it. Delete it.
Ralph