An ITSP (Internet Telephony Service Provider) offers an Internet data service for making telephone calls using VoIP (Voice over IP) technology. Most ITSPs use SIP, H.323, or IAX (although H.323 use is declining) for transmitting telephone calls as IP data packets. Customers may use traditional telephones with an analog telephony adapter (ATA) providing RJ11 to Ethernet connection.
In the United States, net2Phone began offering consumer VoIP service in 1995. Before 2003, many VoIP services required customers to make and receive phone calls through a personal computer.
ITSPs are also known as VSP (Voice Service Provider) or simply VoIP Providers.
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