Symptoms
The Busy On Busy option works as expected for only the first 12 minutes of a call in Microsoft Skype for Business Server 2019. If a call is made after 12 minutes have elapsed, the call is signaled to the call recipient instead of a busy signal being sent.
Resolution
To fix this issue, install the December 2021 cumulative update 6 for Skype for Business Server 2019, Core Components.