Signal-call is not working over VPN


i got my vpn with roadwarrior client running (still have a problem with the fact, that the client has access to the green network and needs a firewall rule), but if i try to use signal on my phone over the vpn of ipfire, i can not make calls, i just can write messages. So whats the point there? Does it need extra ports??

I made “redirect gateway” settings for the client on the server.