Why should Public Telecommunication Operators (PTOs) care about Internet telephony? The short answer is that it is cheap. This makes it a potential threat to regular telephony, the core business of the PTOs. The next section explores the reasons for the affordability of voice over the Internet. Section 4.3 describes how Internet telephony works—what users need, what providers provide, and how telephony over the Internet operates—and what is necessary to make Internet telephony a market reality. The fourth section presents some of the effects of this new technology, especially on consumers. It describes how voice over the Internet could become a mainstream commercial reality. Section 4.5 analyses the different implications for ISPs, while section 4.6 analyses the implications for PTOs and their likely responses.