I currently have iPhone 3GS with AT&T and I have another line without contract with AT&T that my wife uses. IF I take iPhone 4 on that number with $15 plan and switch SIM card from my iPhone 3GS to iPhone 4 and my wife can use my iPhone 3GS. Does that work? I know within same network (AT&T) SIM cards work in different phones. Appreciate you help.