There are supposed to be codes (at least there were once) that 'stopped the clock' (NO REFERENCE OR CONNECTION WITH CURRENT VODAFONE TARIFFS) on some phones that would allow you some free minutes. I think the networks caught on to this and would disable any sim card that was found to be operating with such a code or 'hack'.
I used to have a load of different codes a few years ago when using a (now ancient and defunct) nokia 3330 for various features including the ability to find out other peoples text message contents. I only used this as an experiment beween friends phones and in no way hacked any other mobiles.
Things have moved on now so the networks are a bit more secure so doubt there will be much out there that won't risk disconnection from the network.