Panel Call Indicator, or PCI, is a form of signalling used between two telephone offices. It was also originally called Relay Call Indicator (RCI).
Originally designed along with the panel type telephone office, PCI was intended to allow subscribers in fully automated exchanges to dial numbers in manual offices the same way they dialed numbers in their own exchange. For PCI to achieve its purpose, the panel office sent the requested number to the manual office, where the number was lit on an operator's display. The switchboard operator at a PCI-aware manual office reads the number from the call indicator display and completes the call in the usual way. As a format of interoffice signaling, PCI is one of multiple options retained for compatibility in the #5 Crossbar switch, a later system which served as the platform for the initial DTMF push-button telephone services.
In the British Director telephone system, Coded-Call Indicator working (CCI) filled a similar role, as it displayed dialed telephone numbers at the local manual exchange in a mixed (automatic and manual) linked-number area.
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