git pause) the voice processing system fered by the telephone exchange are intended to replace those validates the access code and returns number unobtainable, engaged offered by on-site PABXs. or ringing tone to the caller, depending on the availability of the Service selection digits (dialled by the caller) are sent immedi- service. ately after the call is answered by the voice processing system from Two or three bursts of ringing tone are given before the caller is the telephone exchange. As in the case of DDI, these digits corre- connected to the selected voice service. (In a PABX environment spond to the last 2 or 3 digits dialled by the caller. However, these the ringing would continue until the extension was answered.) are sent by DTMF tones to the voice processing system. At this point call charging is initiated by the voice processing Once the call is established a recall indication can be sent from the system reversing the polarity of the battery voltage it supplies to the voice processing system to the exchange. This takes the form of a telephone exchange. This means that call charging is initiated by the timed loop break (100ms) or an earth applied to one leg of the equipment, not the telephone network. As can be imagined this telephone cable pair. results in the rigorous testing of the equipment before it is approved The telephone exchange then sends a dial tone to the voice for connection to the public network. processing system as an indication that it can dial out the number using DTMF tones and invoke features such as three party con- ference calls. 37.2.1.2 Normal telephone type When the original calling party or the voice processing system terminates the call all parties are usually released from the connec- The voice processing system mimics a normal telephone. This kind tion. of connection is used for information update lines or when the voice processing system is placed behind a PABX. The telephone ex- change sends ringing current cadences to the voice processing 37.2.1.4 E&M type system. Af