(RS means Radio Shack nearest-equivalent part number)
R1,R2 10kohm,1/2watt (RS271-1126)
C1,C2 .1uF, >200V (RS272-1053)
T1 600ohm 1:1 telephone audio transformer (RS273-1374)
D1,D2 generic signal diodes (RS276-1653)
Telephone tip and ring are the two center conductors of a four-conductor modular telephone connector, and for this circuit it doesn't matter if they are reversed. (RS279-391)
Sound card microphone-input connector is a 1/8" 3-conductor audio male plug. (RS274-284C)
Theory of Operation: C1 and C2 block DC current path, so this device does not bring the phone line to off-hook state, but the capacitors pass AC voice signals. The device D1 and D2 limit ringing voltage that can reach microphone. T1 provides isolation between telephone line and sound card.
