Client behind firewall and proxy

We have an jitsi Installation and it works fine for most of clients. Since we block udp completely on the server it works as designed over 443. One of our client, with advanced security-request has got problem. The web-trafic is only possible via proxy over 443 and 80. When the browser starts or tries to enter a meeting room, this does not work. Wireshark reports on the client side binding request on port 10000/udp. We tried this also with meet.jit.si same result. Can anyone explain me what is going on there. Thanks in advance.
Lorenz