Whether your phone is locked to your network depends on the network you're with and even on which handset you've got and which plan you bought it on.
For instance, all phones bought on EE and Vodafone are locked to their networks. So if you want to use another network's SIM, you'll first need to unlock your handset.
Conversely, when it comes to O2 phones sold on pay monthly plans, only some are unlocked. But if you got a pay-as-you-go phone from O2, all models are unlocked.
If you're with Three, the good news is that all phones they sell are unlocked. That applies irrespective of the model you own, the plan you bought it on or whether you bought it on pay monthly contract or pay as you go.
All sounds complicated, right? Agreed. But the good news is that there's a simple to check for sure whether your phone is unlocked.
All you need to do is to remove your existing SIM and insert another network SIM in your phone. You could borrow a friend's or family member's just to test it.
In the event that your phone accepts the SIM and you've got a mobile-phone signal, your phone is unlocked and you won't need to get it unlocked.
If there's no signal, your phone is locked. If that's the case, you'll have to call your network to ask them to unlock your smartphone and may have to pay a charge.