Phone Call Interceptor
This device connects between the phone line, an answering machine,
and your phones. When a phone call comes in, the answering machine
picks up the call and informs the caller to enter the access code to ring
the phone or to wait for the beep and leave a message. If the correct
code is entered, the answering machine is immediately disconnected while
the device generates phone ringing tones on an external speaker. Picking
up any phone causes the device to return to its initial state.
Since I live in a rather large house (with 5 other people at this time), I used a PA horn for the ringer/speaker. As a compromise to local sanity, the level of the ringer is reduced when I am sleeping or not at home. The 'door sense' input detects the 'state' of my bedroom door since that is where the speaker is located.
This is one of the most useful devices I have made lately. As usual, you can check out both the schematic and source file if you wish to make something like this.