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Aside from giving the owner the chance to search an array of providers, there is a couple of other good reasons why you might like to SIM unlock your cellphone even before you've completed your contract. Should you be heading overseas as an example, you might want to use a local SIM card when you are there to keep away from heavy roaming service fees. Or if you've decided to sell your handset then you'll probably get more cash and a lot more interest in a cellphone which is free to make use of with virtually any network.
There's nothing unlawful about unlocking your smartphone. It's your phone and you're within your rights to use it with any carrier you would like. But, there's a good chance network unlocking your smartphone can end your warranty, so it could very well be best to hold-off until you have finished your contractual obligations before deciding to get rid of your current carrier.
A few networks will not charge for unlocking help while others might charge about $50. It is also worth considering that each networks has varied policies pertaining to how long you've had your regular or PAYG contract for and how to get your code. Most carriers have online forms, but many will advise you to call their helplines. The amount of time it may take to unlock your phone yet again can vary broadly between each network. In the UK, Three for example, will unlock an iPhone in a week, while Tesco Mobile declare it might take up to 45 calendar days to unlock IMEI locked devices.
If you are unsure who to trust, I highly recommend Unlock.Zone. They offer unlocks for all major brands and carriers, have a guaranteed price match promise, but even more importantly, offer a 100% refund policy in the unlikely event they cannot unlock your mobile phone. I've used them myself personally and was really happy with the service I received and ultimately that they successfully unlocked my cellphone.
If you want to find out more, here's a link to their web site: Mobile Phone Unlocking Service