A device for locking a telephone to prevent the unauthorized use thereof is described. The device comprises a baseplate that clamps to the web located between the handset cradles of a convention desk-style telephone base. In one form, a locking plate having a pair of outwardly extending wings is used in combination with the baseplate. The wings extend outwardly to cover the switch pins located in the handset cradles. The locking plate supports a cylinder lock, the cylinder of which passes through an aperture in the baseplate. Located on the lower end of the cylinder lock is a cam. When the cylinder lock is rotated, the cam engages the lower surface of the baseplate and locks the locking plate against vertical movement. This action prevents the switch pins from raising upwardly. In a second form, the locking plate hooks under the lower front corner of the telephone and covers the pushbuttons or dial to prevent access thereto. The other end of this locking plate also includes a cylinder lock and cam that engages the baseplate to prevent the removal thereof.