Personally i use the BETAMAX solutions it's what i find
CHEAPER and i try a lot of provider the only pb is find cheaper clone "
now i use voipcheap"
They have a lot of sites but all of them are from the same company then you have to compare between sites
To find the good service you can go to :
Comparison of the prices and the the rates voip Finarea / Betamax for all the destinations
or to :
betamax price comparison
NB
The price of BETAMAX sites can change
everyday DAY so check the prices with the same frequency
Don't give directly your card number for example you can use a special card for Internet buy (can be used only once) i hear that they can charge the credit card more than one time.
But service is OK quality is good and you can call by SIP , INTERNET or by using a local number in a lot of country
Calls other INTERNET get a connection fee (5 Cents )
SIP, and calls to a local number don't get any.
Luciano