If the Microsoft Teams icon is missing in your Outlook calendar, try the following:
Launch Outlook, choose File and then Options.
Select the Add-ins tab of Outlook Options dialog box.
If the Microsoft Teams Meeting Add-in is listed in the Disabled Application Add-ins list, click on the COM Add-ins box next to Manage to change this to Disabled Items and click Go...
Check the box for the Microsoft Teams....... file, click OK.
Close and relaunch Outlook.
If it still doesn't appear, go to the File > Options > Add-ins
Close and relaunch Outlook for the changes to take effect. Go to your Outlook calendar which should now look like below: