Abstract: An interface circuit for providing power and signal voltages to a telephone line under the control of a microcomputer within the telephone exchange comprises four charge pumps, each comprising a pair of driver/amplifiers which charge a pair of capacitors through a first diode with two further diodes discharging the capacitors to charge a capacitor connected across the telephone line. The four charge pumps are arranged symmetrically to provide positive or negative potential at the TIP terminal and positive or negative potential at the RING terminal. Modulation of the voltages to transmit a desired signal is obtained by modulating the power voltage supplied to the amplifier/drivers. The current drawn by the telephone line is measured by sensing the current drawn from the voltage source by the amplifier/drivers and this is used to extract the received signal from a telephone line and also to extract a measurement of peak current drawn from the TIP and RING leads separately.