need help to unlock corporate iPhone owned and locked by ex-employee

You are dealing with two types of limitations; device passcode and activation lock (Find My Enabled).

For Activation Lock, if the iPhone is listed in Apple Business/School Manager, then you have the ability to prove chain of custody to Apple. Open a ticket with Apple support and request an activation lock removal. You can get started here. If you have access to GSX, you can open a ticket there and provide a screen shot of the asset in ABM/ASM. This is proof enough of ownership.

If the iPhone is not in Apple Business/School Manager, you can still prove chain of custody if you can present to Apple the original sales receipt. See link above to get started.

As for the passcode on the device, if the unit is enrolled in an MDM, you can issue a passcode reset. If you are not using an MDM, then boot the unit into DFU mode to erase it. if you try the passcode too many times, the unit will lock for an increasing amount of time between each failed attempt.

Now, a little guidance on activation lock. If you are using an MDM, I strongly encourage that you configure your pre-stage to prevent activation lock on the device. Use restriction profiles to further lock out the options for Find My. While this will still allow the use of a personal Apple ID, it will prevent an employee from linking the device to their personal Apple ID. If you have devices already linked to Find My, you can request that employees disable that service before there is a separation or incorporate the process into your offboarding process.

You can in a business environment with the proper tools in place BEFORE you need to bypass. This includes Apple Business/School Manager and MDM. With a proper MDM and AxM, you can bypass activation lock if needed.

You can’t bypass activation lock

only the owner can unlock the phone

I actually encourage my users to lock their devices with a personal Apple ID. MDM can escrow a bypass, so both the user and the org can lock down a device that goes missing.