Panel call indicator

Panel Call Indicator, or PCI, is a form of signalling used between two telephone offices. It was also originally called Relay Call Indicator (RCI).

PCI, originally designed along with the panel type of telephone office, was intended to allow subscribers in automated exchanges to dial numbers in manual offices the same way they dialled numbers in panel offices. As a format of interoffice signalling, 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.

For PCI to achieve its purpose, the panel office sent the requested number to a lit display (call indicator) in the manual office. The switchboard operator at a PCI-aware manual office reads the number from the call indicator and completed the call in the usual way.

In the British Director telephone system, Coded-Call Indicator working (CCI) filled a similar role, as it displayed dialled telephone numbers at the local manual exchange in a mixed (automatic and manual) linked-number area.

The usual seven-digit US telephone number consists of a three-digit office code and a four-digit line number within the given office. After a caller in a panel office dialed the office code, the sender looked up the type of the called office (panel, manual, etc.) and whether PCI was to be used. Then the sender found a trunk to the called office. Once a trunk had been found and the decision to use PCI had been made, the office code was not needed. So only the line number was sent to the manual office.

However, for two reasons, PCI always sends five-digit line numbers rather than four (and the caller may need to dial eight digits rather than seven).

The panel system was designed to work with manual offices of up to 10,500 lines. Callers dialed the office code followed by the line number within the office. For lines 10,000 and up, callers therefore dialed the office code and a five-digit line number. For offices with fewer than 10,000 lines, callers dialed four digits but PCI sends a leading 0.

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