Removing Apple ID From Work Phone

An ex staff member signed into his work phone with his apple ID, he has now left the company and we need to give the phone to a new starter, however, we cannot remove his apple ID from the phone.

Everytime we try and reset the phone it prompts us to login to his icloud and ID which we cannot as we do not have his password and no way of contacting him.

How can I remove him from the phone so I can erase all settings and data.

Thanks

Stephanie