Non-branded ES400 Sprint Firmware installation
If you're using a non branded ES400 (i.e. not a "sprint" or "Verizon" ES400) than a firmware upgrade is needed to make the Sprint cell data service to work on a non-branded ES400.
1. Utility to unlock ES400 from AT&T service. Phone is locked when a sim card is inserted.
2. If the ES400 is the newer B models with the new radio that’s switchable, the latest version is: 02.31.0002 which has a release date in Oct, 2011.
Here is a link for the firmware needed for Sprint service (note ROM upgrades are destructive)