See:
http://www1.sprintpcs.com/explore/ueContent.jsp?scTopic=internationalRoaming

In many cases it can be done, but it's generally *far* cheaper (an order of
magnitude) to just get a regular cell phone or pre-paid plan overseas than to
use your US phone.  Of course, the downside with this later approach is that
you end up with a different phone number overseas, but I still think that
unless your the CEO of a multi-national corporation or similar, that's a small
inconvenience compared to paying, e.g., $1.29/minute!