Unlocking your iPhone can be a bit of a nightmare and it depends what network you're on and whether you're still in contract.
You can always buy a SIM-free device from Apple, or one of the box-only options available from the various operators – in other words, it's been sold to you without a contract and without subsidy.
Once you've been informed by your network that your iPhone has been unlocked, you'll need to pop in your new SIM card and connect your device to iTunes to complete the process of the unlock – you can't just do it on the handset alone. iTunes will then confirm the unlocking of your handset.
Be warned that once you have unlocked your iPhone, using it with another network will require new Access Point Name (APN) settings from your new network - otherwise you won't be able to browse and send email.
Check with your new provider as your handset may update rather than you having to type them in manually.
How to unlock your O2 iPhone
Unlocking your O2 iPhone is the most simple of the three major networks and even pay monthly customers can do it at any time.
Simply visit O2's online form and fill it in. The process takes up to 14 days. If you have a Pay-As-You-Go (PAYG) iPhone on O2, you will need to pay a £15 fee which can be deducted from your balance. Be warned that you will lose the free web and Wi-Fi bolt-on that O2 gives you.
How to unlock your Orange iPhone
It's not possible to unlock your Orange iPhone if you're still within the first three months of your contract. After this date, it is possible by paying a small fee - £20.
To do this, simply dial Orange (150 from your Orange handset or 07973 100150 from another number). Orange says the process of the unlock can take up to 7-10 days.
How to unlock your Vodafone iPhone
Vodafone iPhones remain locked to the network for the duration of the contract after which you can visit its Unlock Code Request Form to have it unlocked.
Vodafone says that you have an iPhone unlocked from Vodafone then you can apply to have it unlocked – use the unlock code request form. Vodafone says that it can take up to 14 days for Apple to approve the unlock request.