Extension Phone Line Switcher by SCR 2N5061
Having multiple extension telephones at home is very convenient. You can make or receive phone calls practically anywhere in the house. This circuit disables other telephones connected to the phone line whenever a telephone (either the master or any extension phone) is in use. The circuit is inexpensive and is guaranteed to keep the phone conversation private. The circuit does not need an external power supply. It gets its power from the telephone line.
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http://www.electronicsforu.com/efylinux/circuit/cir45.htm

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