Phones can be locked not sims. Sims are pin protected. You must know that pin.
If you have some other some sim card of a different network just use that and see if you are able to connect. If you don't have alternate sim and want to be certain and want to avoid wait and talking to customer service on phone, just go to any store and they will check for free. I travel abroad frequently and I got my phone checked at Costco and T-mobile store despite having ATT service. They will help you unlock if the agents are available. Or just visit apple store and get quick resolution.
Bring it to apple store and they check it for you
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