There are hundreds of them.
Before we go there, however: You should know that Vicibox.com is the Approved installation method. Later code, non-commercial, no extraneous logos or broken functions (from modified screens). Goautodial is very useful IF you require CentOS. For any other purposes, your best experience with be with Vicibox.com's installer (PDF on the site) and then use the Vicidial Manager's Manual (free on EFLO.net) for configuration (don't skip any pages!).
Next up, choosing a carrier: While I can not share company names (my employer prohibits this for non-paying-clients), you'll find several on this site. When looking, avoid (like the plague, no kidding) any carrier that uses the word "unlimited". As a group, they are to be shunned.
You're looking for simple pay-per-minute and should not pay more than 1.9 cents per minute. It's usually easy to find 1.5 cents. And the goal (for solid quality carriers) is 1 cent per minute. You can also find some under 1 cent, but often they are not real carriers, but "aggregate" carriers that will not complete all your calls. As long as they are up-front about this, they can save you money (and you will use a more expensive carrier as a failover to fill the gap).
Happy Hunting!