Voice over Internet Protocol, which is also commonly referred as VoIP, is a set of technologies that enable voice calls using the internet connection as the transmission medium instead of the traditional telephone landline system, PSTN (Public Switched Transmission Network).
The term VoIP was coined by VoIP forum which was setup in May 1996 to ensure product interoperability and develop a high quality of service for internet telephony products. It is also sometimes referred as IP Telephony because it uses IP protocols to enable enhanced voice communications throughout the organization. The traditional PSTN uses circuit switching where a dedicated channel is setup between two points before users can talk to each other whereas VoIP uses IP protocols to convert the voice signals into digital signals that travels over the public internet or a private data network. A Basic VoIP service allows users to make and receive calls over the internet for free whereas Interconnected VoIP services also allows users to make as well as receive calls from the telephone landlines usually for a monthly service fee. The dedicated VoIP phones are used for interconnected VoIP services but landline phones can also be used using a special adapter.