Panel call indicator

Panel Call Indicator, or PCI, is a form of signalling used between two telephone offices.

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.