Have you tried Jajah.com??
Jajah.com is a free web phone calling system and makes amazing promises: no downloads, headsets or phone adapters.
They have got $3 million in funding from Sequoia Capital in 2005, and now they have a million paying customers. For each customer, Jajah is making $10 monthly, and these fees subsidize the free calls, and how do they do this? They have cut deals with many telecoms for access to cheap local last-mile connections, and the long-distance part of the call goes over the Internet.
To avail of this system, you simply use your existing phone; you enter it on Jajah website, and then you enter your the number you want to call, either local or international. At present, you need to have Internet access, but Jajah plans to take the computer out of the call process.