Telephony image #1 Example of how an IP Telephone System Works Telephone Network System

In telecommunication, telephony involves the general use of equipment to provide voice communication over distances, specifically by connecting telephones to each other.

Digital telephony is the use of digital technology in the provision of telephone services and systems. Nowadays, almost all telephone calls are provided this way, but sometimes the term is restricted to cases in which the last mile is digital, or where the conversion between digital and analog signals takes place inside the telephone. Telephony was digitized to cut the cost and improve the quality of voice services, but digital telephony proved useful for new network services (ISDN) to transfer data speedily over telephone lines.

IP telephony is a modern form of telephony which uses the transmission control Protocol/Internet Protocol (TCP/IP) protocol popularized by the Internet to transmit digitized voice data.

Computer telephony integration (CTI) enables computers to know about and control phone functions such as making and receiving voice, fax, and data calls with telephone directory services and caller identification. The integration of telephone software and computer systems is a major development in the evolution of the automated office.  A new generation of applications (middleware) is being developed as a result of standardization and availability of low cost computer telephony links.

Like all categories of an institution's ICT system, it requires energy to run and costs money to purchase and maintain. For this reason it's important to measure its energy use and how many tasks it can perform when procuring for a new phone system.