There are a number of reasons why you might want an unlocked phone or tablet. Maybe you want to use a different SIM card when abroad for cheaper calls on texts, or want to switch networks.
Or perhaps you've purchased a phone online and want to know if it's locked to a particular network, or want to make sure it's unlocked in order to sell it.
If your phone or tablet is locked, you will only be able to use a SIM card from the mobile network it's locked to. This can be very frustrating if you need to use a SIM from a different network only to find that your phone won't let you.
If you purchased your phone SIM-free (and you've purchased it brand new rather than second-hand), it's almost definitely going to be unlocked to allow you to decide which SIM you want to put in it. However, buying one on contract from a phone shop or a network might mean that it's locked.
Locked phones are much less common now than they used to be, and it is much easier to unlock them if they are locked, so there's no need to worry if you discover it is.
It might cost you a small fee, and sometimes you'll need to wait until your contract is up to unlock the phone, but that will depend on the network the phone is locked to.
How to check whether your phone is unlocked
If you have the phone on your person - whether it's an iPhone, Android, Windows Phone, BlackBerry or something else - then the easiest thing to do to check whether your phone is unlocked is by trying different SIM cards in it.
You could borrow a SIM card from a friend or family member that is from a different network to the one you've been using, and pop it into your phone to check whether you have any signal. If not, it's likely that your phone is indeed locked.
We'd recommend restarting the phone before checking the signal, as it sometimes takes a restart for the card to be picked up by the device itself.
A good way to determine whether the new SIM card is working is to try making a phone call. If it doesn't connect, it's likely that your phone is locked.
If you don't have the phone because you're buying it, you'll have to ask and trust the seller to find out. But even if it does turn out to be locked there's an easy fix in most cases so it's unlikely to render your new phone useless.
You may find apps claiming to be able to tell you if your phone is unlocked but we would avoid using this method as they can't necessarily be trusted. Simply trying different SIM cards is your best option.
If you find that your phone is indeed locked, follow the links below to go to the unlock page for your network.
Alternatively use a third-party unlocking app like DoctorSIM.