Carrier lock Sim Locked

See here - How to unlock your iPhone for use with a different carrier - Apple Support

Find your carrier from this list - Wireless carrier support and features for iPhone in the United States and Canada - Apple Support

Then request an unlock from their support page. I don't know which carrier you have, so I don't know the specifics.

Interesting. In that case, I would say to contact Apple Support - Contact - Official Apple Support

Follow this link - Contact Apple for support and service - Apple Support

Select your country/region, then dial the number they show. Explain to them the issue, and follow their instructions.

Well, it should be the one you purchased service from.

If you don't know, go to Settings > Cellular and look at the carrier name.

You can try Apple on Twitter.

Apple on Twitter