You should always check the IMEI of an iPhone before you try to unlock it and uncover any hidden issue that might ruin the process.
An iPhone IMEI Check will expose:
The SIM Lock Status
The Locking Carrier
The Blacklist Status
The iCloud Lock Status
The Contract Status
The Financial Balance Status
The Warranty Status
The Replaced IMEI/Serial Status
The iPhone’s original specs
As you understand, this info also makes iPhone IMEI Check the best way to check a used iPhone before buying it.