Can I unblock a network blocked iPhone 4s?

I know it might not be moral/legal but is it possible to unblock an iphone 4s that has been blocked by a network?
I have heard about using a jailbreak to unlock the carrier setting, I know this works for a phone locked to a network for example you can jailbrake an o2 iphone to work with 3, but would this work for a network blocked handset?

thanks,

Added (1). I'm looking for the answer to whether or not it can technically be done.
Eg would the handset work if it was jailbroken although displaying 'no service' with a sim card installed in it.

It is illegal to do that. The only way a phone gets blocked is if it is reported stolen. If the owner finds it then it can be unblocked but not anyone else. The networks themselves would be breaking the law if they allowed this as the phone is stolen goods.