A locked phone means yu can only use it on the network you bought it on, but this can differ depending on which network, handset and plan you have.
For example, if you buy a phone with EE or Vodafone, it'll be locked to their networks. So if you want to switch over to another network's SIM, you'll have to unlock your handset first.
But when it comes to O2 phones bought on pay monthly plans, you'll find that some are completely unlocked. But all pay-as-you-go phone from O2 are unlocked.
Three phones always come unlocked. Regardles of the type of phone, price plan it was bought on or if you got it on pay monthly contract or pay as you go plan.
Don't wory if youre a bit confused, it's all abit complicated. However, there's a simple way to see if your phone is unlocked.
All you need to do is to take out your current SIM and put another network's SIM in to your phone. Just ask a friend or family member if you can borrow theirs for a quick test.
If your phone takes the SIM and it works like normal with a signal, this means your phone is unlocked and so it won't need to be unlocked at all.
But, if you can't get a signal, this means your phone is indeed locked. You'll then have to call your network to ask them to unlock your smartphone which may incur a payment.
For more info on switching networks, check out our handy guide