A system where multiple telephones are used by businesses in an interconnected fashion that allows for features like call handling and transferring, conference calling, call metering and accounting, private and shared voice message boxes, etc. A business phone system can range from just a few telephones in a small business up to a complex private branch exchange (PBX) system utilized by large businesses.
Business telephone systems can function over the Public Switched Telephone Network (PSTN) and/or over the Internet (Internet telephony or VoIP). Business phone systems can also be delivered as a hosted service (typically referred to as a centrex), which can free companies from having to invest in costly equipment.
Recommended Reading: The Difference Between VoIP and PSTN Systems.