Sure, as long as none of the apps you want to run needs a phone connection. (IOW, you can't do texting, but you can still use the web.) Activating the phone is a change the carrier makes on its servers, it doesn't do anything to your phone. (Right now, if you try to do anything but place an emergency call - 911 in the US, 112, I believe, in Europe - the carrier's servers look your phone up, don't find it, and say "sorry, emergency calls only". When the carrier changes things on the server ["provisions your account"], the server says "Oh, that's John - he has an account with us - go ahead".]