This could be due to network quality. Please check the Teams Admin Center:
1) Open Teams Admin Center > Users > Find the User/Users having issues
2) Click on the Meetings & calls tab
3) Scroll down past "Recent Meetings" and go to Past Meetings as seen below and find the call that was having issues:
4) Check the Audio quality & click on Network to see if there were any issues with the call (with regards to network connection). This will most likely be the reason for quality degradation on Teams voice/video calls.
If this is helpful please mark as correct answer.