We have recently found out about a solution for paying less for our international phone calls.
This is http://www.nonoh.net/.
Basically all you have to do is to register as a user and buy credits (in multiples of EUR 10).
Then, you just input in the site form the number where you care calling from and the number where you are calling to. Then, your phone will ring, you will pick up and then the other number will be dialed and you can start talking.
This way, you do not pay anything on your phone, as you have been called. The only fee you pay is the rate/minute that is displayed on the nonoh website when you input the 2 phone numbers.
For example, we are using our fixed line to call from Luxembourg in Romania and we pay only 1 cent per minute if we call a fixed number of 6 cents per minute for a mobile number. That's incredible cheap.