I have used this type of circuits for quick testing of telephone equipment. This is a quick hack that does not exactly resemble the characteristics of the real telephone network, but is enough for this kind if applications. If you want better simulation, change the resistor to 600 ohm and increase the power supply voltage. And by the way you can also feed signals to telephone if you connect the circuit output wires to small amplifier output. +12V (well filtered power supply or batteries) | | --- | | | | 220 Ohm 2W resistor | | --- | || +---------||---------------------------> audio input signal | || Capacitor 470 nF | | Telephone | | +--------------------------------------> audio input ground | | power supply ground Circuito original em: www.hut.fi