iPhone compatibility
Will my iPhone 6S that was sold to work on Verizon’s network work in Italy with a SIM card change?
If I understand your question correctly:
If it is an Unlocked & SIM free iPhone then it should work, check & ask your Carrier, they will know it.
See this Apple article: How to unlock your iPhone for use with a different carrier
How to unlock your iPhone for use with a different carrier - Apple Support
-> Apple can't unlock your iPhone for use with a different carrier. Only your carrier can unlock your iPhone.
If you sold that iPhone, have you read this Apple article? -> What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch
What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support
Then see -> Check your Apple ID device list to see where you're signed in
Check your Apple ID device list to see where you're signed in - Apple Support
-> remove it from your account.