Bought a used iPhone. Where can I use the IMEI to learn more?

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

"You can find out if your iPhone is locked by going to Settings > General > About. If "No SIM restrictions" appears next to Carrier Lock, your iPhone is unlocked."

"Apple can't unlock your iPhone for use with a different carrier. Only your current carrier can unlock your iPhone."

You’re here, asking this, which usually means the seller disappeared so no refund (or was sold as-is), which usually means you bought a brick.

The All Too Common SAD Reality of Buying … - Apple Community

Your carrier will be able to tell if it’s blacklisted.

For the carrier lock: How to unlock your iPhone for use with a different carrier - Apple Support

If the parts in your iPhone were not original, you would be notified about an Unknown Part.

If money is still owed on this iPhone, it’s likely that you won’t be able to use it with other carriers as you must pay off the remaining balance with the original carrier.

Also where did you purchase this refurbished iPhone from?