Failing VOIP connections to Microsoft Teams meetings over WiFi

When users run Teams meetings over WiFi for either location (one has a 60E and the other a 100D) the presentation part goes fine, but the (5 or so) dial-in voice connections are often intermittent. 

I've read that QoS and WNN need to be turned on because the VOIP gets a lower priority, but the CLI Reference says that's the default anyway.

I know I'm using a lot of bandwidth (a Teams meeting in HD can take 2 to 8 Mbps), but I'm not seeing anything that suggests why we are having problems.

My ability to test this is limited, so I thought I'd ask if anyone has seen something similar. It may have nothing to do with the firewall. I just don't know.

Thanks.

Dave