Verizon would not. For AT&T and T-Mobile (as well as MVNOs based on those two such as StraightTalk, SimpleMobile), you can waltz in (or go to the website) and

Verizon would not. For AT&T and T-Mobile (as well as MVNOs based on those two such as StraightTalk, SimpleMobile), you can waltz in (or go to the website) and get just a SIM card to pop into the phone of your choice. When it comes to data service, there can be frequency differences that could result in some phones having to fall back to 2G service instead of 3G or 4G, such as my wife's iPhone 3GS on T-Mobile.