Least Call Routing
Just as many telecommunication carriers employ IP telephony in their backbone networks, several operators sell IP capacity (often combined with termination services) as a least-cost routing solution for international calls. Given the increasing prevalence of this type of transmission, many callers are probably already making IP telephony calls without knowing it. Pulver.com’s "Minutes Exchange" acts as a "market maker" between parties originating and terminating IP minutes.

Call Termination
Hundreds of small companies, many related to established ISPs, offer international call termination in almost every country in the world (including many where IP Telephony is prohibited). Some of these firms are full-service clearinghouses and offer billing services as well. The barriers to entry are low and so are the prices. See, for instance, the list of termination rates around the world available on the IPxStream web site, or that of Arbinet-TheXchange, where the prices quoted are often below the settlement rates, which would otherwise apply. IP telephony minutes are being terminated on the PSTN just about anywhere the Internet reaches.

Source: ITU