Can you unlock iPhone to any carrier?

Yes - if your phone was locked on purchase - there may be conditions you need to fulfill like paying for it in full

You will need to speak with your carrier

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

Apple doesn't unlock iPhones. Ask the carrier it's locked to; some don't offer this, and others set requirements.

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Just because it is paid off does not get it unlocked. You need to ask the carrier to unlock it.

Owning it outright and being locked to a carrier are not the same.

Look under Settings to find your carrier.