Group should be notified of "busy" status for a user when a group call is initiated by that group to a user who is already on another group call

When user is on a group call and another call is initiated from another group to the user , the user can see the call from other group. In such instances user should be marked busy, and second group should be notified that the user is busy as well. This is already available at the user level but customer wants this functionality at the group level.