To restrict external Teams communication for both calling and personal chat to only organizational users that use the microsoft.com domain, you need to use the Teams admin center to configure the external access settings. Here are the steps to follow:
Sign in to the Teams admin center with your Microsoft 365 username and password.
In the left navigation, go to Org-wide settings > External access.
Under External access, turn on the toggle for Users can communicate with other Skype for Business and Teams users.
Under Cross-tenant access settings, click Manage settings.
In the Cross-tenant access settings pane, under Calling and chat, select Allow specific domains and then enter microsoft.com in the box below. This will allow your users to communicate with Microsoft users via calling and chat.
Click Save.
Note: You also need to make sure that microsoft.com has configured their external access settings to allow communication with your domain. See Communicate with users from other organizations for more details.
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