A dial device for a telephone set having a function of preventing toll dialing, in which a resistor or a zener diode is connected in series to a dial impulse contact of the dial device while a series connection of a first diode and a control circuit is connected in parallel with the resistor or the zener...http://www.google.de/patents/US3976846?utm_source=gb-gplus-sharePatent US3976846 - Dial device for a telephone set having a function of preventing toll dialing
