Iphone activation lock linked to old email

I have one of our old staffs phones that I have restored and now has the activation lock coming up.

The users email address they used as their apple account username no longer exists and cannot be access.

The staff member has given us his previous details and I have successfully changed the apple account online from the old unused account to a new email account / username that we can access online.

However the phone is still wanting the old user name and password to activate..

I have read on other threads about temporarily changing the username back to the old email address, however this is no longer possible as it wants me to enter a verification code that it is trying to send to the old email address before it will change the email account name.

Any suggestions on how to have this phone activated?