The Microsoft Office suite of clients offers integration with the default chat app, historically Skype for Business. This allows a user to interact with other users on that platform from within the office application, typically when co-editing, but largely throughout Outlook in features such as contact cards.
When you’re in a migration scenario, especially if you’re in Islands Mode, you’ll want to have Office pick one client over the other. You do this from within the Teams Client.
Click on your photo in the top right, then settings, then tick the box shown below:
And once you’re done this, make sure you restart Office. You may also need to wait a few minutes for the setting to take effect.
Note that if you switch to Teams-only mode, this setting is asserted for you – no need to manually configure it.