Carrier or network lock
A carrier lock restricts which mobile networks a phone can use. The carrier that applied the restriction handles eligibility and unlocking. Contact that carrier and check its policy; an IMEI may help the carrier identify the phone, but does not decide Apple account ownership.
Apple Activation Lock
Activation Lock is tied to Find My and the Apple Account associated with the device. It appears during setup when the device is still linked to an owner. A carrier unlock does not remove this protection, and removing Activation Lock does not necessarily change carrier status.
How to check a used iPhone
Ask the seller to erase the phone and remove it from their account. Start setup before paying. Confirm that it reaches the Hello screen and does not ask for a previous owner’s Apple Account. Separately, ask the carrier whether the phone is eligible for use on your network.
If you already bought one
For Activation Lock, contact the seller or previous owner first. For a carrier restriction, contact the carrier. Keep the two cases separate and avoid services that promise one IMEI check will clear both.