This circuit is extremely simple, therefore there is less chance of any problems. It can be placed anywhere on the phone line and it will record any conversation on any phone on that line.
470 Ohm 1/4 Watt Resistor
1K 1/4 Watt Resistor
100K 1/4 Watt Resistor
6K 1/4 Watt Resistor
0.01 uF 100V Ceramic Capacitor
24VDC Reed Relay
Wire, Headphone Plugs, Phone Plug Or Alligator Clips
1. Email Derek Tombrello with comments, questions, etc.
2. To use the circuit, simply connect it in series to the phone. Then plug in the recorder. It will start recording when the phone is picked up.
3. The values of R1 and R2 may have to be adjusted, depending on the characteristics of the phone line it is connected to (they vary). If the R1+R2 combination is increased sufficiently (try 2.47K) then the circuit can be connected in parallel with the line, thus covering all attached devices. (Thanks to Mark Wright for the suggestion).
Source: Electronics Lab