Sim locked iPhone

If the previous phone was locked to a particular carrier, that carrier lock is in force until the carrier releases. As of now you can only use service from that carrier.

You need to talk to that carrier to ask its release, but most likely only the original owner can arrange that. Some carriers never unlock and you can ONLY use service from that carrier. Others require you to have active/never-late-paid service for 6 months before it releases.

Apple does not override a carrier lock, but it does sell phones that are not locked to any carrier from the get-go.

There are 2 things here. The SIM is the card your carrier gives you. So if there is still a SIM from the previous owner in it, ask them for the code or remove the SIM card and do the one you own yourself with your own phone number in it.