I understand. For a basic setup, meaning one VOIP terminal at your end and a provider, you could use the service of the company providing this board.
Look in the banner in top of the page. If you buy an ATA 186, setup and account with Dual Talk and you will be talking within few minutes from opening the box.
Prices may not be absolutely the lowest for any destination, but they are good across the board.
Imagine that I need to pay 7 cents a minute on the U.S. just to call someone next county.
VIa London and Dual talk I cut that down to less than 3 cents.
Absolutely mind twisting.