armpogart,
I think Switzerland ISDN is slightly different from the Italian one here, but from what I understand , thet device is a Network Termination for service provider , it translate the signal coming from the outside (the green cable, here is called the “U” interface) to one ISDN BRI S-bus (the one you call the VoIP phone but I think it’s not, it should be an ISDN phone) and three analogue PSTN line
Since I don’t know the swiss standard and don’t know if any “swiss ISDN user interface to VoIP gateway” exist, I suggest to use a common ISDN BRI VoIP gateway such Mediatrix 4400 series I use succesfully with no problem.
Simply connect the “S” output to the Mediatrix and the Mediatrix ethernet to eth. switch/FreePBX and do the routing.
P.S.
Check: The TWO connectors you indicates with arrow as 1st phone number (one to ip and one to analogue phone) should instead go both to TWO ISDN devices as those are marked as “S” bus, that is the BRI interface and the ports are connected in parallel inside the board.