Sim not supported

If you get an alert about the SIM card in your iPhone - Apple Support - https://support.apple.com/HT201420

Find out what carrier your phone uses: Settings > General > About > Carrier

Then see if your carrier allows unlocking:

How to unlock your iPhone for use with a different carrier - http://support.apple.com/HT201328 - Only some carriers offer the ability to unlock iPhone for use on any carrier. Follow the instructions in this article to unlock your iPhone.

Your iPhone is locked to a different cell provider. You can call AppleCare and ask them what carrier it is locked with. Then you will need to contact that carrier and see if you are eligible to unlock the iPhone and what the process is.

Hey there,

Welcome to Apple Communities,

I can help you there, your phone carrier may not be supported by the phone you have, as phone models differ phone bands, which means you may not have a phone that supports the carrier, if you could post the iPhone Make; e.g; iPhone X, 8 Plus, 8 etc and the carrier you are on we might be able to help you more,

Harry

Um... no. That has nothing to do with it. the phone is carrier locked.

In that case, I would go to the carrier and see if they have stopped supporting the phone, KiltedTim

First they will have to determine what carrier it is locked with and then try to unlock it, supporting it has nothing to do with it. And it has already been suggested.