Abstract: A modular digital telephone system and method featuring a universal telephony shelf adaptable for incremental expansion of legacy telephone switches and as a building block in the construction of new telephony switches. The shelf includes identification and connectors for shelf pairing, and is adapted to receive redundant power supplies, plural line cards, and either a peripheral shelf interface unit or a switch array card which provides both a local shelf interface and three interface units for connection to the peripheral interface units of other shelves. System wide fault displays are provided.