Not sure if you figured the 3G 2G part or not so.....

To get it off 2.5, go to
settings -> mobile network -> network mode and change from there.

You have dual, 2G only or 3G only, I personally have my phones set to 3G only as 2G is ancient tech, internet 3G vs 2G is like broadband vs dial up.
Battery standby does benefit from 2G mind you.

Try choosing between 2G 3G on iOS or Android, you cant.