Under Settings/Wireless Networks/Mobile Networks, turn off 3G data, data roaming, mobile networks, uncheck all boxes. (That is, turn off 3G mobile data.)
Under Settings/Call Settings/System Select/Sprint Only.
If that doesn't work, search the web for and try installing the app called "Roaming" by Nick Dupree. Set the Roam Preference to "Home Only".
By the way, if you get a free (US-based) Google Voice number and load a SIP app like GrooveIP Lite (free) then whenever you have a WiFi Internet connection (anywhere in the world, including the US) you can:
1) Make free calls to anywhere in the US & Canada.
2) Receive free calls if people call your US-based Googe Voice number.
3) If you add money to your Google Voice account, you can also make international calls at Google's cheap rates.
