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With normal internet protocols, you can have a network consisting of many routers (20 - 30 or more) which can result in a very high round trip time (RTT).
In addition to standard IP, VoIP uses the real-time protocol (RTP) to help ensure that packets get delivered in a timely way. Using public networks, it is currently difficult to guarantee Quality of Service (QoS). Better service is possible with private networks managed by an enterprise or by an Internet telephony service provider (ITSP).
As of this writing, VoIP is still plagued by lack of generally accepted interoperability standards.