Simplex on Fedora

I thought I would like to give Simplex a go on my Fedora laptop so I
installed it with Gnome Software from Flathub. Version 5.8.2 of
Simplex.

I can connect to Simplex on my phone and send text messages back and
forth, but audio calls don't work at all, nor video calls.

For example if I make an audio call from my phone to my laptop, Simplex
pops up and ask if I want to answer the call, after pressing Accept a
tab opens up in Firefox and asks if I want to allow microphone access,
after accepting nothing much happens after that. There is just a black
tab in Firefox with the text "starting... contact has e2e encryption"

Anyone that can make audio calls with Simplex on Fedora?