cal.com requires a one-time, organization-wide admin consent to function, as organizations are typically configured to restrict users from consenting to third-party applications.
Here are the steps to authoriza cal.com with Outlook:
Log into the Microsoft Entra admin center as a Global Administrator or Application Administrator
Navigate to Identity → Applications → Enterprise applications and look for the cal.com application
Important: If the app is not listed, ask one user to attempt the sign-in flow once. This will trigger the creation of the Enterprise App object in your directory, even if the login fails
Select the cal.com application and go to Permissions in the left menu
Click Grant admin consent for [Organization Name]
Accept the permissions (strictly limited to
Calendars.Read,Calendars.ReadWrite,User.Read, andoffline_access)