Audio from a telephone line can be obtained using a transformer and capacitorto isolate the line from external equipment. A non-polarized capacitor isplaced in series with the transformer line connection to prevent DC currentfrom flowing in the transformer winding which may prevent the line fromreturning to the on-hook state. The capacitor should have a voltage ratingabove the peak ring voltage of 90 volts plus the on-hook voltage of 48 volts,or 138 volts total. This was measured locally and may vary with location,a 400 volt or more rating is recommended. Audio level from the transformer isabout 100 millivolts which can be connected to a high impedance amplifier ortape recorder input. The 3 transistor amplifier shown above can also be used.For overvoltage protection, two diodes are connected across the transformersecondary to limit the audio signal to 700 millivolts peak during the ringingsignal. The diodes can be most any silicon type (1N400X / 1N4148 / 1N914or other). The 620 ohm resistor serves to reduce loading of the line if theoutput is connected to a very low impedance.