I'm a cingular customer and although their overseas per minute rates have come down over the years, they still charge for both outgoing and incoming calls. If you buy an unlocked quad or tri-band gsm phone, you can purchase pre-paid sim cards for wherever your travels take you. And, in most countries, you pay only for outgoing calls, not incoming calls. I call my office, family, etc then have them call me back. It's usually much, much cheaper. Please check before purchasing international sim cards that incoming are free. There are a few countries that practice the highway robbery tactics of USA carriers and charge for both incoming and outgoing calls. For unlocked tri and quad band gsm phones and prepaid sim cards I've used both ustronics.com and telestial.com. Sometimes one is having a better deal in what I need.
For those who already own "locked" gsm phones go to travelinsider.com (a good travel info resource). There is info on how to unlock your current gsm phone. Costs about $5.00 I think.