After the update to iOS8, the switch to disable the 3G has been replaced by a disable 4G switch.
4G is already off but how could I also disable also 3G (which is still active, as shown on the status bar)
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Settings->Cellular->Voice&Data: choose 2G
You can only really turn off "all" mobile data, so there's no falling back on good old GPRS...
If you go to:
Settings -> Mobile -> Mobile Data (and 4G if you want that off too).
"Mobile Data" is the old "3G" switch, I guess. I suppose apple assume that the entire world is 3G ready xD
It will disable all data via mobile (not wifi etc..)...