. These private exchanges are connected to their local exchange by a group of exchange lines and the public exchange selects a free line within the group (PBX hunting) when a caller dials the first line of 1209 Hz 1336 Hz the group. In some cases calls incoming to the PABX are routed to an operator (attendant) who makes the connection to the required line (extension). Direct Dialling In (DDI) may also be provided so that the caller is able to dial the required extension. APABX can also be provided as a virtual group within the public exchange, a system 697 Hz known as Centrex. 31.2.3 Hierarchy of exchanges 770 Hz Each local public exchange provides the means for setting up calls between its own customers' lines. It is normally also connected to other local exchanges in the same area (e.g. the same town) by 852 Hz junctions. For calls outside this area, junctions are provided to trunk (toll) exchanges which are in turn interconnected by trunk lines, enabling calls to be established to customers in other areas. Trunk lines are also provided to one or more international exchanges 941 Hz which enable calls to be established to customers in the national networks of other countries. A single exchange can sometimes combine two or more of these functions and the larger national networks have more than one level of trunk exchange in their hierarchy (e.g. three levels in the UK, four Figure 31.7 Push-button (multifrequency) dialling in the USA). A trunk call in the UK, therefore, passes through up to six trunk exchanges in addition to the local exchanges at each end. In large areas, some local exchanges may be interconnected only by having calls switched through another local exchange, a tech- nique known as tandem switching which can be independent of the 31.2.1.7 Ringer (bell) trunk switching hierarchy used for calls outside the area. (See In the idle state, the handset is on its rest, the gravity switch is open Figure 31.2.) and no direct current flows in the line. The ringer is, therefore, designed to operate on altern