If you're looking to get a new SIM plan for an old phone - whether its yours or one you bought secondhand - you could find that it is locked to a network. This is where a phone has had a SIM in it and now can only use that network.
While that sounds permanent, it's actually easily fixed. First, try putting a SIM card from another provider into your phone. Make a call and if you can't then its locked. You can also get in contact with your network to find out if its still locked.
How to unlock a smartphone:
Unlocking a smartphone is either a breeze or a bit more complicated depending on the network its locked to:
EE Once six months have gone by on your contract, you can call EE on 0800 956 6000 and pay them £8.99 to unlock your phone. It says it will take around 10 days to complete. PAYG phones can be unlocked for free. Both iPhones and Google devices unlock for free after a certain time with EE.
O2 Whether you have an iPhone or Android device, pay as you go or on a contract, O2 can unlock it for free. You simply need to log into the O2 app, select unlock my phone and you're good to go.
Three Fear not, all phones on Three are unlocked as standard. Shove whatever SIM you like in there, it will work a treat.
Vodafone The red network doesn't quite get the same marks as Three, as your phone will be locked to them on arrival. But they have made unlocking handsets absolutely free within 10 days of your request.