Need the Desktop app to integrate with on-prem Exchange and Outlook (for Contacts)

Using the Desktop app the only options I have under Settings | Calendars and Contacts are Microsoft (365) and Google. We don't use either of these. Our organization uses an on-prem Exchange server and Outlook. We want to access our contacts using the Desktop app, too! We have the Outlook add-in, but we'd like to "advertise" to our employees that they can do everything communication-wise from within the Desktop app.