I want to change the sim card in my mobile phone, because my calls are so expensive. You can buy these cards online for about £10 which is cheap, but it says with them:

"NOTE:
Compatible with most mobile phones that are not SIM locked to a specific network."

So, my question is how do I know if my phone is locked to a certain network? Is it as simple as just buying the card and taking mine out and putting it in, would it work straight away?