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VoIP providers may be viewed as commercial realizations of the experimental Network Voice Protocol (1973) invented for the ARPANET providers. VoIP can be a great benefit to major corporations trying to cut costs by simply running network cables. Although it requires those using VoIP to have a well configured network it is a new use of IP that is extremely cost efficient. Skype and Vonage are great examples of how VoIP is being greatly utilized world wide.
Voice-over-IP systems carry telephony signals as digital audio, typically reduced in data rate using speech data compression techniques, encapsulated in a data-packet stream over IP.
There are two types of PSTN-to-VoIP services: Direct inward dialing (DID) and access numbers. DID will connect a caller directly to the VoIP user, while access numbers require the caller to provide an extension number for the called VoIP user.(Wikipedia)