iPhone blocked on network
Phones are only blocked on this level for two causes that I'm aware of:


The phone was stolen and the owner reported it stolen
The phone was subsidized, and the owner stopped paying for it


A stolen phone block will generally never be released, though it depends on the carrier. Some carriers may when presented with a police report or other similar paperwork indicating that the property was recovered, or a payment of the outstanding balance on the device.

There is no way to unblock it yourself. Blocking basically means that the cellphone network looks for the IMEI of your phone, and if it sees it, refuses to service it. Since IMEIs are generally burned into the phone at a very low level, they are usually not changeable.

I'd inquire with Vodafone and get more information as to why they blocked the device. If it's stolen, you want to get rid of it, preferably back to its original owner.

If its a billing issue, you may be able to pay the amount due to get service restored.