Assume that you are set up for a Team Call group and enabled for the Busy on Busy feature in Microsoft Skype for Business Server 2015. If another user tries to call you when you are active in another call, you do not receive an incoming call notification, and the user receives a busy signal. This means that the Busy on Busy setting is not ignored for a Team Call group. The Busy on Busy setting should be ignored because the Team Call group has a higher priority.
To fix this issue, install the August 2021 cumulative update 6.0.9319.619 for Skype for Business Server 2015, Front End Server and Edge Server.