Hi @ewald ; thanks to you, I struck the solution! For me this worked:
New rule=
Resulting in.
(10.0.2.54 is my SPA112)
I tested incoming and outgoing calls just fine!
EDIT: just to be clear; I have no idea about the security implications of the above rule. It is working for me in a GREEN/RED configuration.
Thanks to everyone, especially @manfred_knick for chipping in to help here!