I see @Thomas_Eichelbu , so you have the GW as the default route for your traffic.
Do you use QOS? could it be that from the office, traffic is not prioritized in any way up to the CKP GW exiting Internet ?
(as I don't know your design, I can't say much more)
In our case, the majority of complains were related to big meetings (like you said) and all others were OK.
Still what I can say, is that in our case, where we have over 20 (or more) users from same place, they complain. We have a meeting happening every Friday with over 150 users from around Europe, not many sharing same office/place, and all is good there. But others where we have multiple sharing the office, come back with Complains .
And like I said, in our design, Teams traffic doesn't pass through CheckPoint GW, so we're suspecting MS doing some stuff....
Have you tried also to look over the codecs used for Voice/Sharing ?