The key hole behind what 's telephone
Switch long-distance business, where you can lock long distance, that is, not unlock, can not make long-distance calls!
In the long-distance telephone lock, dial "0", "0" key circuit with a switch circuit is cut off, once the dial tone signal sent after the start of the closed, with a low potential to control the switch circuit to dial "0" button is switched on, so in second after dialing numbers, usually "0" can be successfully issued. But if it is the first number of "0", because at this time the switch circuit is not connected, the telephone can not dial "0", that is, the international and domestic long-distance first "0" can not be issued, can not make long distance calls.
The key hole Shisuo long, lock to get through long-distance calls, local calls only.The principle is that when dialing, the dial - up circuit sends a "closed tone" signal (usually a low potential) to turn off the send - to - receive call.
