@joseph no this is not spam.
I tried both on your demo server and on my own test VPS to get nextcloud talk to work. The calls only work when both callers are on the same network which tells me that the turn server is not working. I also have a hetzner storage share nextcloud instance in which talk works out of the box in the same test scenario.
The hetzner nextcloud talk settings show their turn/stun server configured to port 443 and it works great.
I was wondering if I was doing something wrong, and have been looking through forums to try and see if there is a fix. So far it sounds like the built in cloudron turn server is not on port 443 which means it doesn't work reliably across all networks.