Depending on when you purchased your i Phone or i Pad, it may be locked to the Vodafone network.
You can unlock your i Phone or i Pad from the Vodafone network by performing a backup and restore or updating to i OS 7 or later.
SIM locking is very common if subsidized phones are sold with prepaid contracts.
It is important to note, however, that the technology associated with the phone must be compatible with the technology being used by the network carrier.
This business model allows the company to recoup the cost of the phone over the life of the contract.
Such discounts are worth up to several hundred US dollars.