Tomi, first of all "thank you for coming back to this special issue". Unfortunately I'm way behind your level of knowledge:

How can I "debug network topology" between the tunnel endpoints N9 and my vpn server FRITZ?
Ifconfig, route (completely unknown to me!), ping and traceroute must be run on which device/PC? All on N9? If N9: please the complete comand line as I do have some basic knowledge of linux but am no expert - thx.

EDIT: SOLVED by changing one parameter of the SIP/VoIP configuration

FINAL THANKS AGAIN TO TOMI FOR VPN9C !!!