Sim Card and international travel help

[markppc6700]

I was wondering if any1 tried using a sim card from another country ex "china mobile or 3 network" in asia. I am leaving on a business trip in 2 weeks and i need to make sure that there is a way to change my sim and still be able to make calls without ROAMING on sprint's network.

It was a big selling point for me when i found out this phone had a sim card and it was a world phone but i am trying to test it with a TMOBILE sim and it still shows Sprint on the screen and i am still able to call my voicemail by pressing 1 which leads me to believe that it is still on sprint's network.

I tried using some of the methods in the manual but that's a waste of time.

thanks in advance