You can find some voip related articles on the http://www.radvision.com site. Also you can search for ‘VOIP’ on the internet for articles on VOIP. VOIP basically offers long distance service on the packet network. The tarrif for the service is implementation specific and if implemented the VOIP gateway itself will do the billing depending on the amount of time the call is active etc.,
Suppose if you have a VOIP connected on the Internet, you can dial from the PSTN and hook onto the VOIP gateway and then dial the remote user you want to talk to (the remote user also should have a similar setup on the other side). So by making a local call from the PSTN to the VOIP you are able to speak over a long distance, but you pay only for your local call. Thatz the advantage of the VOIP. The Voice quality you may get may not be equal to that of the quality you get on the PSTN network. But it will not be that bad also.