Is it possible to do some packet sniffing. Like tcpdum or so.

I have some trouble with sip accounts and need to see, why I get no
telefon connection, even when the sip-accouns ar already registraded
and online.

You can perform a network capture from the Balance after logging in by typing in this URL:

http://<Your device’s IP>/cgi-bin/MANGA/support.cgi

From there you can start, stop, and download a capture and this will allow you to view SIP packets in detail using Wireshark.

Wow. That´s the point.