Could be Teams holding an active audio connection to headset - quit and re-open Teams.


Or;

As a work around can disable the Skype dial tone and howler sounds in the sound properties;

  1. (for Windows 10) right click sound icon in system tray
  2. select sound properties
  3. Select the Sounds tab.
  4. In the Program Events section, scroll down to locate Skype for Business.
  5. Under the Skype for Business node, select Dial Tone - also set same for the Howler event
  6. In the Sounds section, scroll to the top of the list, select None, and then select OK.
  7. Screenshot that shows setting the Dial Tone sound to None.




From: https://learn.microsoft.com/en-us/skypeforbusiness/troubleshoot/server-conferencing/sfb-play-dial-tone-teams-call

Or: https://support.bluejeans.com/s/article/Skype-for-Business-can-play-phone-dialtone-sounds-when-placing-BlueJeans-calls