as the poster above said, the USB phones basically just function as another sound card so there is no reason why it should not work with any VOIP provider and Video or IM programs (like GoogleTalk and MSN).
I have never tried VOIP Raider... will give it a try now seeing as some of you guys are recommending them. I have always been happy with Skype... have given them thousands of Rands of business over the last few years.
Just one other note, some of these guys work through Paypal, which is fine, I have used paypal a lot. BUT, paypal have been known to lock people's accounts for "suspicious" behaviour reasons, that happened to me once years ago. Just be aware that you should not trust Paypal any more than they trust you. They are fine in terms of their financial transactions but their treatment of customers and security measures are draconian and never in favour of the customer. If they label you as "suspicious" then that is it, you don't stand a chance. Just one thing to be aware of. Rather work through one credit card company if you can. In fact, I suggest you get one card that is ONLY for online transactions with a low credit card limit.