how to unlock my iphone

Your iPhone sounds like it's locked to another carrier. To unlock your iPhone from this carrier you'll have to follow the steps given in this Apple Support Guide.

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

Your device is locked to a carrier and doesn't accept SIM cards from other carriers. Your device can be unlocked only by the carrier that it is locked to, if you're eligible.

Contact the carrier that the iPhone worked on previously. Ask that carrier about the requirements for unlocking it.

Axel F.