Syncing 4-digit user extension to WebEx Calling using Entra Wizard

We are working on migrating to WebEx calling. We setupt the Microsoft Entra ID Wizard. It is mapping all of the standard fields like First Name, Last Name, Title, etc

I can't figure out how to synch a users 4 or 5 digit extension. I don't see it in the supported field mappings on Entra or Webex, so I assume it's not supported. Or do we have to use on prem AD to achieve this?