Many services require a minimum amount of minutes before you get the best rate. Others round out fractions of a minute to the next highest, others charge by the second. Still others give breaks for calls from or to special locations. Ask for the lowest prices and promotions. Whatever you choose, you won't get the lowest rate if you don't ask for it.
Many programs offer signing bonuses or frequent-flier miles.
Consider billing the calls directly to your credit card. If you've got a credit card that offers bonus points or frequent-flier miles, use it. There are even greater incentives if you're willing to receive your billing statements through the Internet. Know the 1010 codes. If you know how to use the 1010 system, there are hundreds of unadvertised specials each week.