Zoom disables Do Not Disturb somehow

In the middle of a class I'm teaching on zoom, a call will come through and disrupt the class, stopping the sound and video until I decline the call. When a zoom meeting is not running, the same call will be diverted to voicemail. I've set Do Not Disturb to block calls from everyone, no exceptions. Someone in another thread suggested the problem was fixed by disabling call waiting, but that didn't work for me. The call still comes through and disrupts my meeting. Any help or a fix would be appreciated!