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If you turn off 3G you'll still have access to GPRS, Edge data. Turning off Cellular Data turns off cellular all data.
 
More specifically, turning off 3G will disable ALL access to the 3G (UMTS) network, not just data.

In some areas you might get a 3G signal where there isn't a 2G (GSM) signal to make a phone call/send a text message.

Some countries (like Japan) don't even have 2G networks anymore.
 
when i'm at home where i have a wifi connectivity, i turned off both 3g and cellular data (gprs and edge), so the battery life lasts much longer.
 
im in canada on telus and I tried this but when I shut off 3g it just goes to no service. Kinda lame.
 
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