This is very useful in a situation were there is lots of noise and it is impossible to hear the phone, such as a workshop. Here is such a device.
1. You may need to use a lower voltage zener for D1 and D2.
2. You can use a regular relay instead of a solid state relay, but the arcing accross the contacts may destroy it pretty quickly.
3. Be very sure that you have not accidentally connected 120V to the phone line when building and installing this circuit.
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