Cause
Proper audio device not selected in Control Panel and/or Skype for Business settings.
Resolution
1) Go to Control Panel > Sound and Audio Devices > Audio tab.
2) Under "Sound playback", change the Default device to your computer's sound card/speakers. Change the Default device for "Sound recording" to your Skype for Business phone.
3) Go to the Voice tab.
4) Changed the Default device for "Voice playback" to your Skype for Business phone. Under "Voice recording", change the Default device to your Skype for Business phone.
5) Click OK to save your changes.
6) In your Skype for Business options, go to the "Audio Device" section.
7) Choose your Skype for Business phone as the device you want to use for audio calls.
8) At the bottom of the "Audio Device" section, uncheck the option to ring phone and speakers at the same time.
9) Click OK to save your options.
OUTCOME
The problem will be resolved.
Your enterprise voice phone calls change the Windows system volume to 0 or 100, and the volume stays there even after disconnecting the voice call.