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.
   
Part                         Total Qty.              Description

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   

Electronics Circuit Application

Please note: I have received several emails saying that this circuit will not work and that it may hold your line off hook and to me it looks like it will (it would put quite a load on the phone line). For some, it has worked fine. Build at your own risk.