Previous chapters described some of the advantages that Voice over IP (VoIP) can offer over traditional circuit-switched telephony, such as lower cost of network implementation, integration of voice and data applications, new features, and potentially reduced bandwidth for voice calls. It would be nice if all telephony were to be carried over the Internet Protocol (IP) so that these advantages could be made available globally. Unfortunately, that day will not arrive for a long time. Replacing all traditional circuit-switched networks is not feasible, and perhaps not even desirable. Among other reasons, the cost would simply be exorbitant. Rather, one can expect a gradual evolution from circuit-switched to IP-based networks such that both types of networks will exist side by side for a very long time.

Given that VoIP networks and traditional circuit-switched networks will have to coexist for many years, they will need to interwork as seamlessly as possible. In other words, users of existing circuit-switched systems should be able to place calls to VoIP users, and vice versa. In each case, the user should not have to do anything radically different from simply calling another user on the same type of network. The challenge therefore is to develop solutions that make such seamless interworking possible.