Internet Telephony Service Provider. A supplier of digital telecommunications services using Voice over Internet Protocol (VoIP). The Internet connection is established using IP phones or a Private Branch Exchange (PBX) system using Internet Protocols (IP) like SIP and H.323.
An Internet telephony service provider (ITSP) offers digital telecommunications services based on Voice over Internet Protocol (VoIP) that are provisioned via the Internet.
ITSPs provide services to end-users directly or as whole-sale suppliers to other ITSPs.
ITSPs use a variety of signaling and multimedia protocols, including the Session Initiation Protocol (SIP), the Media Gateway Control Protocol (MGCP), Megaco, and the H.323 protocol. H.323 is one of the earliest VoIP protocols, but its use is declining and it is rarely used for consumer products.
Retail customers of an ITSP may use traditional analog telephone sets attached to an analog telephony adapter (ATA) to connect to the service provider's network via a local area network, they may use an IP phone, or they may connect a private branch exchange (PBX) system to the service via media gateways.
ITSPs are also known as voice service providers (VSP).[citation needed]