All right, good to know. I have set up a port forwarding for the port specified in the template. Why I changed the port? → Good question, for a better conscience I usually change the default port.... No idea if this is so useful.
By the way, in the meantime I grabbed two cell phones (cellular) and made a call (video and audio) and sent some messages. Works.
I also tried the whole thing with the default port (port forwarding, calls, messages). Same behavior.
I'm still surprised that turns does not work for me.... Nevertheless, many thanks to you, @ich777.