If I Disable 3G, EDGE Also Goes

On my iPhone 4, to save battery during the evenings and nights when I need the phone to fetch less data as compared to the comp, I turned off Enable 3G in settings (and turned off Wi Fi too). But instead of simply reverting to EDGE, it loses the data connection completely – the net is inaccessible. I have use cellular data on, data roaming also on, for good measure, but no luck. Rather than turn on and turn off the 3G each time, I don’t mind having the net on EDGE for a while. But EDGE is now apparently not an option.
Any tips? Thanks, Gary.
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Dhruv