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.
Parts
R1
470 Ohm 1/4 Watt Resistor
R2
1K 1/4 Watt Resistor
R3
100K 1/4 Watt Resistor
R4
6K 1/4 Watt Resistor
C1, C2
0.01 uF 100V Ceramic Capacitor
K1
24VDC Reed Relay
MISC
Wire, Headphone Plugs, Phone Plug Or Alligator Clips
Notes
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
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