Don't you have to plug it back into iTunes again or wifi? If you can skip so you can get to the dial pad and do *228 option 1. Use another phone to call if you can. If not working go to a store.
Go to Settings -> General -> Reset -> Reset Network Settings.
the phone will power cycle and should automatically start activation on its own. that doesn't erase your content from the device, it only resets the cellular portion(and the wifi passwords..)