Is unlocking your mobile phone illegal?
When you buy a pay monthly or pay as you go mobile phone in the UK, it is usually locked to the network that you bought it on. If you want to change your network and keep your phone you’ll find it impossible - when you put the new network SIM card in, it will not work.
Thankfully there are many solutions out there - for a start, you could buy a cheap unlocked pay as you go mobile legitimately from companies like Carphone Warehouse or Phones4u. You can also get your locked mobile phone unlocked - usually in dodgy little shops in all major cities.
But isn’t this illegal? Many people believe it is, but the answer is actually no - its perfectly legal. Here’s a quote from the Metropolitan Police:
In some instances, when a person purchases a mobile phone, the airtime provider (ie. Vodafone, Orange, etc.) will sometimes ‘lock’ that handset to its own particular network.
If that handset is then required to work on another air time provider’s network, in the case of a legitimate resale for example, it will therefore be necessary to ‘unlock’ the mobile phone from its original airtime provider.
Consequently, ‘unlocking’ is a legal activity and involves the use of software on a computer or a machine called a flasher (electronic device used to remove information held in electronic circuits) to unlock the phone from a particular air time provider’s network.
It is commonly done in mobile phone shops, markets, and car boot stalls.
You’ll also find a full breakdown of the legal arguments on a BBC article entitled Unlocking the Locked Phone Debate. The case is not so clear cut in the USA as it is here in the UK.
So go forth and unlock. Be aware that unlocking your phone will almost definitely void any warranty you have though, so do it only when you really need to.

Thanks for the useful info. It’s so interesting