Hi tarnend.  I'd love to know what FG Support recommends.  A downgrade for us is not possible at this time.  I am not sure if we can get around it by using a split tunnel.  I was going to test that when we hit a maintenance window.  Or switch everyone to a Windows Native setup which in testing allows Teams calls to go through.

I am experiencing the same issue with Teams running through a full tunnel with Windows.  The calls will not connect.  If I disconnect the FortiClient VPN the calls function properly.  Did you ever find an answer to this issue?