You'll need to take a close look at your cell phone, calling habits and future travel plans to find the cheapest way to make international phone calls.
What Type of Cell Phone Do You Have?
Your first concern should be the type of cell phone service you currently have. U.S. cell phones typically use either the 850 megaherz (MHz) band or the 1900 MHz band, while cell phones in most other countries operate on the 900 MHz or 1800 MHz band. If you have a U.S.-only phone, you will need to buy or rent an "unlocked" GSM phone, either a tri-band or quad-band phone, to be able to make phone calls while you're overseas. A GSM phone uses at least one of the 900/1800 MHz bands and will therefore work in other countries. Japan and South Korea are special cases; you'll need a UMTS 3G 2100 MHz phone to make calls from these countries.
Tip: Whether you buy or rent a phone, be sure it will operate on the band your destination country uses. Some tri-band phones only offer 900 MHz or 1800 MHz, saving the other two bands for U.S. compatibility. If you plan to travel abroad fairly regularly but don't know where, exactly, you plan to go, think about buying a quad-band phone so you will have access to cell phone service wherever you go.