Reference:
https://docs.microsoft.com/en-us/microsoftteams/teams-app-setup-policies

Explanation
https://docs.microsoft.com/en-us/microsoftteams/meeting-policies-in-teams

Explanation

See the solution in the explanation below:
Explanation
To prevent incoming Teams call notifications when users are already on a Teams call or in a Teams meeting, you can use the Teams admin center to enable the Mute notifications during meetings and calls option. This option will suppress the incoming call notifications without rejecting the call. The caller will still hear the ringing tone and the callee can still answer the call from the Teams app or the taskbar. Here are the steps to enable this option:
Sign in to the Teams admin center with your Microsoft 365 username and password1.
In the left navigation, go to Teams > Teams update policies.
Select an existing policy or create a new one by clicking Add.
In the Update policy pane, under Meetings and calls, turn on the toggle for Mute notifications during meetings and calls.
Click Save and then Close.
Note: You can assign different update policies to different users or groups of users. For more information, see Manage update policies in Microsoft Teams2.