A PABX (private automated branch exchange) system allows a single access number to offer multiple lines to outside callers. PABX systems are installed at companies, call centers, and businesses both small and large. PABX systems are available in four modes - digital, analog, IP, and hybrid. PABX systems provide features like digital extensions, caller ID, welcome messages, data recording, and voice mail. The document also provides contact information for VRS Technologies.