Corporate iPhone locked by o a fired user's personal Apple ID. How can I use or reset the phone?

If you can’t gain the cooperation of the ex-employee …

The phone will need to be reset using this procedure:

If you forgot your iPhone passcode - Apple Support

When finished, the phone will most likely be “Activation Locked.”

Since it’s corporately-owned AND you can provide the original purchase invoice, it might seem that an Activation Lock Support request would be viewed favorably if submitted.

See link near bottom of this article:

How to remove Activation Lock - Apple Support

You’ll need to have the person whose credentials locked the phone unlock it.