ve short fixed length messages, or for inwards, normal telephone, Centrex and E&M. it to prompt the caller for an interaction (DTMF, Grunt etc). If the caller does not reply the system drops the call SO as to be ready for new callers. Alternatively, if the network / returns dial tone 37.2.1.1 Direct Dial Inwards (DDI) to the voice processing system when the caller hags up, a dial tone detector can be used. However to be effective the dial tone detector This type is widely used by the Audiotex service providers and is must reliably detect dial tone in the face of the voice processing currently offered in the UK by BT using the Derived Services system replaying speech. Digital signal processors are suitable for Network (DSN) and Mercury using their normal network. The BT the filtering and detection function in this case. DSN has been dimensioned to cope with the large volumes of traffic that can be associated with Audiotex applications. The telephone network forwards the last 2 or 3 digits dialled by the caller to the 37.2.1.3 Centrex type voice processing system. These digits select the opening service given to the caller. This circuit type only supports incoming calls. This is a variant of the normal telephone type. The more popular It has been used with PABX systems in the past to allow callers to large capacity PABXs support the connection of voice mail systems dial straight into a particular extension telephone on the PABX from through interfaces of this kind. Whilst not being described as Cen- the public telephone network. trex, the interface is very similar. These interfaces differ from The service access digits are sent by decadic dial pulsing from the normal extension ports in that DTMF tones identifying the calling telephone exchange to the voice processing system. When the party (if its an internal call) are sent by the PABX as soon as the call digits have been completed (detected by a quiet interval signifi- is answered by the voice processing system. Centrex features of- cantly longer than the inter-di