Persistent Poor Audio/Video Quality when Speaking on MS Teams meeting despite ample bandwidth

I participate in MS Teams meetings for my job, but I am not the administrator of the Teams account (nor do I work for the organisation that pays for and administers the Teams account).

The default is for all participants to participate on video and audio, with occasional screen sharing.. 

Several times recently (including on fairly serious meetings, like Board meetings and job interviews) I've had others tell me that they couldn't hear me / my connection was bad.  [ At that point, turning off video hasn't helped much if at all. ] As a work-around, I've had to dial in for audio (but that's a bit embarrassing during, say, a Board meeting).  Sometimes I get a message from Teams telling me that my internet connection is poor (or words to that effect), but usually I hear complaints first from other participants, or I can see video problems myself.

My broadband is the fastest service that Deutsche Telekom will sell me at my address.  My router is a FritzBox!7590. I am not using a VPN during these calls.

When I have run speed tests on my connection, the downlink has never been less than 200Mb/sec, and it's usually closer to 250.  Uplink has never been less than 30Mb/sec, and it's usually 50Mb/sec. I've checked right after meetings, including meetings with problems, and have started doing random tests during similar hours.

I have tried using my MacBook (a reasonably current model) and an iPad Pro (running nothing but Teams during the meeting), and the problem has occurred on both.  All devices are running the current released operating systems. I have made a point of rebooting them before calls, but that hasn't made any difference.  [ the router has also been rebooted several times recently, but not necessarily before every call ]

During calls, I disable all other devices in my house (it's just me here, so I know no one else is streaming or gaming on my connection, but of course I don't know what may be happening elsewhere in my neighbourhood, and contention may be possible -- but again, my speed tests suggest my service remains more than adequate).

I've so far had no similar problems with Zoom.  In fact, before one important meeting, I got the organiser to do a test with me, but she choose to use Zoom (she thought we were testing my internet quality) and our test was fine -- but during the later Teams call, I had to drop off and rejoin by phone, which was extremely awkward.

This "should" work, and it does for most of the participants (others occasionally have issues, but I have by far the most). It's almost like Deutsche Telekom throttles my connection when I start to speak -- I'm not paranoid enough to believe that, but that's how it appears.

Because I am a regular attendee to these meetings, but am not part of the organisation that actually hosts the calls, they see it as "my" problem to solve.  I could probably approach their administrator to ask for assistance, but if I do, I need to have a plan with a crisp set of requests of them.

Does anyone have any suggestions or troubleshooting tips?  I have not found anything in the forum that matches my situation closely enough to suggest a course of action.