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\3- poleoutletA'? /tt# ^ ^fe^S; I ISelectingbarSelecting Sixthbar barA.position Normal Traffic FacilitiesC-positionB-positionInletFig. 2 X 2647Schematic diagram of 30-line crossbar verticalInternal callsInternal calls are initiated in the ordinary way by dialling the wanted number.A two- or three-digit numbering scheme may be used, according to thecapacity for which the exchange is equipped.Every extension with 0 as last digit can be given priority facility by simplestrapping in the exchange.A priority extension can enter a circuit on which an internal call is proceedingby dialling 2 after receipt of dial tone. A warning tone is sent on thecircuit to indicate that a third party is connected.The priority extension can now deliver his message or, if he wishes to speakto the called party alone, may request both parties to replace their handsets.A new ringing signal is thereupon sent automatically to the wanted extension.SLA SLB90ext. Q f. f*Fig. 3A and B stagesExternal callsClassification of extensionsThe extensions may be divided into four categories as regards external trafficfacilities:1. Unrestricted extensions, which, in addition to calls within the local area,may also dial their own trunk calls.2. Trunk-barred extensions, which may dial calls within the local area buthave restricted facilities for trunk calls (see Extra Traffic Facilities).3. Semi-restricted extensions, which cannot make outgoing calls but canreceive incoming calls.4. Fully restricted extensions, which are debarred from all external trafficand can make internal calls only.Outgoing callsTo make an outgoing call the extension dials a code digit, e.g. 0, for connectionto the public exchange, followed by the subscriber's number.If there are several exchange line routes, each route can have its own codedigit.Outgoing calls may also be made with the operator's assistancetch 1LV)Baa sa^ aasSNK \-Enquiry and transferWhile speaking to an external subscriber, an extension can make an enquirycall to another extension. For this purpose he dials one impulse followed bythe wanted extension's number. The enquiry call cannot be overheard by theexternal subscriber.tch 3LV)FDR-CY-If desired, the extension can transfer the call to the other extension withoutthe operator's assistance.Enquiry or transfer to an operator can be effected by dialling, for example,9.Fig. 4Extension line terminations on the multiplefield90 extensionsIncoming callsIncoming calls from the public exchange are received by one or moreoperators, who extend them to the wanted extensions by means of a keyseton their consoles (fig. 16).73