A modem or other data handling apparatus for connection to a transmission line includes an inductance simulation circuit for providing a low d.c. resistance to allow circulation of a line holding current, whilst presenting a high a.c. shunt impedance to the audio data tones on the line. The simulation circuit comprises two tranistors connected as a high gain amplifier across an input resistor in the modem/line coupling circuit amplifier via a capacitor and the amplifier applies tone signals across the input resistor to augment the tones thereacross. The conductive transistors also act as a low resistance connected across the input resistor for the passage of the line holding current. The simulation circuit acts, in effect, as an inductance, without the use of inductive components. The input resistor is effectively connected, in series with two zener diodes, across the line, and the series circuit receives the line voltage and produces therefrom positive and negative power supplies for energizing the entire modem. No further power supply is necessary.