No sound when 2 or more users on jitsi app

Hello all. For several months I have been unable to use Jitsi on my phone when more than 2 users are present. The sound disappears in both directions when a third person connects. I discovered today that this is not true when connected through the mobile 4G network rather than my home wifi network. I suspect a firewall issue but jitsi works fine when using the same wifi and my pc. For any French users my isp is free.fr and I have a freebox revolution. My phone is a samsung s9. Any information on what ports are used when using the app rather than the desktop version would be appreciated.

Thanks all