I don't think so, the 3G is part the celluar signal. You could use wifi my cell signals always a bit iffy at home, as I live in a rural area but with a good wifi cloud next door thanks to the trendy business park they have built I use wifi
well you can shut off 3G & edge but still be able to make calls. I'm just looking to do this because sometimes I don't feel like being bothered with calls n texts but still like to use Internet.
There is probably something in Cydia, some sort of call assistant/manager if you are prepared to trawl through the thousands of apps to find a really good one.