IPhone 4 not recognizing sim card?

My iPhone 4 is not recognizing the sim card. I went to setting and selected About and it says the carrier O2 so I bought an O2 sim card but still not working, on the top left corner of the phone says No service. When I insert the micro sim card in the phone it says searching but after seconds it says No Service could some one help me to fix this thank you.

Your iPhone is most likely locked to Optus or someone you can get them unlocked at the apple store or search on gumtree for people that unlock phones

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