The convertor has an analogue-digital counter which is actuated from an initial counting state determined for the immediate control of conversion from digital to analogue and for the further conversion from analogue to digital. In the case where a train of predetermined call signal fits is sent to an input register when a subscriber's unit is in a state ready for receiving and transmitting analogue signals, the counter is set for digital-analogue conversion. The counter is reset to its initial position at the moment when its associated subscriber unit is in its rest state.
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