line including a detector for detecting a predetermined start of communication signal a means for coupling the transmission line on the one hand to a transmit line to provide outgoing communications to the transmission line and on the other hand to a receive line to receive incoming communications from the transmission line the detector is connected to the transmit line application_number US 86380001 A description in the following drawings same references represent same elements as in fig1 for clarity only those elements necessary to the understanding of the present invention have been shown 0022 fig2 is a block diagram of a circuit 20 according to the present invention for exchanging communications via a transmission line 6 a modem 4 has an output and an input respectively connected to transmit and receive lines 8 and 10 via amplifiers 14 and 16 the transmit and receive lines are coupled to transmission line 6 by a coupling means 12 according to an aspect of the present invention a start of communication signal detector sd 22 external to modem 4 is connected to transmit line 8 indeed the coupling means which couples the transmission line to the transmit and receive lines is formed by a transformer whereby the communications transmitted by the exchange center arrive on the transmit line the transformer 12 includes a primary winding 23 a connected to the transmission line 6 a first secondary winding 23 b connected to the transmit line 8 and a second secondary winding 23 c connected to the receive line 10 accordingly the start of communication signal is also present on transmission line 8 and it is possible to detect it there the.