I believe that Voipbuster also is another company called Free Call see
FreeCall
Both use either the same or very similar software.
I have Voipbuster ,Free Call and Skype to call home from the USA,with a small amount of credit I can phone a UK or Spanish landline for free.
I also like the Firefox / Skype link facility that automatically turns any telephone number on a web page into a Skype link number.
But going back to the original post, if they are ALL the same company what are they trying to accomplish buy having three similar companies competing with each other?