Why does my iPhone have no service?

I have an iPhone 4 which I haven't used for a while, I tried using a few different sim cards in it and it keeps searching for signal but then saying no service, it is updated to the latest software and I'm wondering if this can be fixed?

What did they say when you called them?

It could be locked to a specific network which SIM you haven't tried as yet. If it is locked then only a SIM from that carrier would work (assuming the SIM itself is activated)

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