No Service when sim inserted into iphone 4?

I just got a used iPhone but when I put my sim card in it keeps saying no service and the contacts which were saved to my sim card aren't showing up. I have turned it on and off again many times and it still isn't working.

The iPhone used to belong to my dad and then my brother before me and they were on different networks so I don't think that's the problem

Added (1). my sim is virgin, my dads was o2 and my brothers was giff gaff

Added (2). So apparently it is the network lock which is the problem so does anyone know how I would go about unlocking the network legally (not jailbreak)

It may make all the difference. What networks? Unless you are on the same network as one of the networks that previously did work, I suspect that is the problem.
I assume your SIM card is still on an active phone line?

Giff Gaff uses the O2 network - it sounds as though your phone is locked to O2…

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