New options for call handling when a user is busy are availalbe and can be set now using PowerShell. The option will eventually also be availalbe via the Teams Admin Center in Calling Policy.
Call handling when a user is already in a call is called Busy on Busy and by default is off in the Global Calling policy.
Previously we could set this to on or off. When on callers got a busy signal and when off, the call just rang. Going forward we also have an Unanswered option which means when the user is busy the setting for unanswered calls is used.
Users configure their unanswered call handling themselves from Settings within Teams.
Users have the option to direct their unanswered calls including to their voicemail or alternative contact, including their own mobile phone if stored in their profile.
To set busy on busy option in the Global calling policy to use the users unanswered calls setting run this script
Set-CsTeamsCallingPolicy -Identity Global -BusyOnBusyEnabledType "Unanswered"
Note that the Teams Admin Center will currently show busy on busy as off once you have done this until the updated control is available.
As with all PowerShell scripts, you run them at your own risk. So I’d always recommedn checking the official documentation before running any script you get from a blog (like this one). The cmdlet used here is Set-CsTeamsCallingPolicy (SkypeForBusiness) | Microsoft Docs
For more on the Admin settings for this feature, please see https://docs.microsoft.com/microsoftteams/teams-calling-policy#busy-on-busy-is-available-while-in-a-call
For more on user settings, please visit https://support.microsoft.com/office/manage-your-call-settings-in-teams-456cb611-3477-496f-b31a-6ab752a7595f