I dont understand.
If you turn off 3G, dont you lose the ability to send and recienve calls and texts?
If not, I honestly had no idea you can do them over wifi?
No you do not.
You can text while talking over a phone on edge and always have been.
With WiFi running data services are shoved over that network so does not effect calls. With WiFi and edge you get better battery life than running on 3G. Hell WiFI + edge takes away all the advantages 3G has over edge.