In Microsoft Skype for Business 2016, consider the following scenario:
You have a USB headset which has a feature used to hang up, hold, or unhold Skype for Business calls.
You have two calls in progress, and you use your headset to hang up one call.
In this situation, you can't hang up or unhold the remaining call by using the headset.
After you hang up the second call, the headset doesn't send proper USB events indicating an additional call is still on hold.
To fix this issue, install the September 1, 2020, update (KB4484501) for Microsoft Skype for Business 2016.