Abstract: A lock-out plug is provided for preventing unauthorized use of a telephone by preventing connection thereof to an operative telephone jack. The lock-out plug comprises a compact plug fitting having a size and shape to fit snugly into an open receptacle of a standard modular type telephone outlet jack. An angularly set threaded bore is formed within the plug fitting such that a forward end of the bore is disposed within the telephone receptacle and a rearward end of the bore is exposed to the exterior of the receptacle when the plug fitting is seated therein. A set screw is received within the bore and includes a forward end locking tip for binding engagement with a side wall of the telephone jack to lock the plug fitting in place and thereby prevent unauthorized connection of a telephone to the jack. The head of the set screw is accessible from the exterior of the jack preferably by engagement with a specialized tool to advance or retract the locking tip relative to the jack side wall.