Abstract: To prevent unauthorized or prolonged telephone conversations, a limiting device is attachable to an ordinary telephone set by means of a padlock means with removable key. The device comprises a timer which can be set only when one of the shut-off pushbuttons of the telephone set is depressed. A knob is provided for setting the timer which is enabled to rotate a rotatable member of the timer only after the knob has been depressed. The knob is arranged such that this depression also involves the depression of one of the pushbuttons. The timer is also operative to depress either pushbutton as the preset conversation time expires. With such a device, it is not possible to reset the timer or retard the movement of the timer in order to prolong the conversation time, because such an attempt would unavoidably result in one of the pushbuttons being depressed on the telephone set and the communication being therefore interrupted.