e detects current flowing in this wire and responds by the ringing signal is available at the PABX. This condition arises from the nature of ringing signals as they are bursts of a.c. signals applying the exchange battery across the pair with dial tone super- followed by a silent period. It is during this silent period that the imposed. The PABX can then go into the addressing state like loop calling and send the digits to the exchange in either 10pps loop exchange line may be seized simultaneously by the public exchange and the PABX trunk circuit. disconnect or DTMF signals. On an incoming call to the PABX the To initiate a call the PABX trunk circuit will seize the exchange public exchange applies an earth potential to one leg of the pair (the opposite leg to that used by the PABX to call the exchange) and the circuit by closing a contact and looping the line. The public ex- PABX detects this signal as a calling signal. change will respond by returning dial tone superimposed on the d.c. exchange battery feed within a specified period of time depending Ringing is also applied by the network but is now of less import- upon the network specification. At this point the PABX can go into ance and the PABX does not need to wait for the ringing signal the addressing phase which can be either loop disconnect signals at during the silent period before acting on the incoming call signal. 10pps or DTMF signals. In either case the out pulsing of digits is Either end can disconnect the call by breaking the loop, but the controlled not by the users telephone but by the system software of PABX is required to run a timeout and maintain the PABX trunk the PABX SO that features such as short code dialling etc. can be busy to all other PABX users until the public exchange has had time to clear the call down and return the line to its idle condition. implemented in the PABX. The address signalling is therefore regenerative signalling and PABX can have 10 pps, DTMF, or proprietary signalling from the PABX telephones to the PABX and 