iPhone carrier locked even with closed paid in full account- no help SOS

Paying off the phone doesn’t automatically unlock it, you need to submit a request to AT&T to do so. If you already did, can you confirm that it says “No SIM Restrictions” in Settings > General > About > Carrier Lock?

See How to unlock your iPhone for use with a different carrier - Apple Support for more information.

Is the locked iPhone less than 60 days old?

att-quietly-changed-its-smartphone-unlock-policy

Carrier Lock:- Request your iPhone’s original Carrier to unlock your iPhone and if they agree and do it, then you may know the further process here --> How to unlock your iPhone for use with a different carrier ...

To be able to use any SIM across the globe you must have "No SIM Restriction" against Network Provider Lock. Please refer to the pic below.

Carrier Lock:- Request your iPhone’s original Carrier to unlock your iPhone and if they agree and do it, then you may know the further process here --> How to unlock your iPhone for use with a different carrier ...

Device Unlocking Policies | Verizon Privacy Policy

Unlock a Phone or Device - AT&T Device Unlock Portal

Unlocking your Sprint device - T-Mobile

FAQs about unlocking your Sprint device

Unlock Your Phone for Spectrum Mobile

I suppose your carrier might ask you to reset network settings and then turn the iPhone off and back on.

Reset iPhone settings to their defaults - Apple Support

You could ask them about that.

This will not release a carrier lock if the carrier has not already released it on their end. And it will make the phone forget WiFi networks - you’ll need to enter passwords for them all over again.

What did AT&T say when you contacted them?