I want to format an iPhone but it stills asks for old credentials
I'm in the IT department for a corporation. Users leave the company without giving us password or resetting their iPhone. I've updated and restored a phone but it still is asking for their Apple Login credentials. I can't contact the user. We've removed the phone from MFM and unenrolled the phone. How do I get the phone to format and go back to factory so I can give to another employee?
[Re-titled by Moderator]