The POTS transport

As you already know, POTS is a transport that runs through the  circuit- switched PSTN. All transport lines  in the  PSTN have  a circuit-identification number, which is either all numeric or alphanumeric. For example, a POTS telephone number has  an area  code, a prefix, and  a suffix that correspond to the  physical circuit and  the  lines that make up that circuit.

Understanding How the PSTN Supports VoIP

No other network in the  world can compare to the  reliability of the  U.S. switched network. (Granted, a handful of disasters have  disrupted PSTN services in specific regions, but  these are the  exceptions, not  the  rule.) Such reliability, however, comes at a high price: The cost of the  switched network, particularly recurring (per-minute) charges, is the  highest in the world. Regional  toll and  international calling  using  the  PSTN are the  most highly  reg- ulated switched network services. This means high regulatory fees in addi- tion to recurring usage charges. But VoIP greatly reduces and may eliminate these types of charges.

How VoIP and the Internet Fit the CSI Picture

To understand how the  Internet relates to the  five CSIs, it helps to first recog- nize what  the  Internet is. No doubt Bell would  be captivated by the enormity of what  we call the  Internet. By definition, it is a network of networks. But just  like VoIP can run on any of the  five CSIs in varying  degrees of quality, the  Internet can also  be accessed from any of the  five CSIs in varying degrees of quality and  security.

VoIP over Internet
Yes, the  Internet is a network of networks, and  the  Web is one of its largest applications. But the  Internet is also  a network that is accessible through all five CSIs.