Simple Phone Tap

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. 

R1           1              470 Ohm 1/4 Watt Resistor        

R2           1              1K 1/4 Watt Resistor     

R3           1              100K 1/4 Watt Resistor 

R4           1              6K 1/4 Watt Resistor     

C1, C2    2              0.01 uF 100V Ceramic Capacitor

K1           1              24VDC Reed Relay          

MISC     1              Wire, Headphone Plugs, Phone Plug Or Alligator Clips 

1. 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.
2. 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).
3. T.A. Betts offers this suggestion for getting this problematic circuit working: "If you drop the resistors and add a 2.2uF capacitor the line will not stay off hook. It blocks the DC from the moniter position."

ref: aaroncake.net/Circuits/phonetap.asp