I am having no luck with telephony/voip and a vr900v.
The VDSL network connection is OK with IPv4 over DS-Lite and apparently native IPv6.
I have a german provider (einsundeins/1und1) which is one of the known brands listed in the pull-down menu for telephony.
I have an old telephone connected.
Whenever I try a call, I get an engaged tone.
When I call myself, I am told that I am not reachable at the moment.

The telephone works fine if I reconnect my old modem (fritz 3692).
I check the connection from phone to vr900v. I have a dial tone and the fritz box knows the phone is there. I can use the dial codes to do things like switch WiFi on and off.
I check my dial-in authorisation using the old box. Everything is known and works.
I tried connecting to "1&1 Internet". There are not many parameters and not much room for mistakes.
I tried connecting to "others" and putting in parameters manually.

I went to the system log and set it to debug level. I see lots of confirmations every 10 seconds that PPP is alive, but absolutely nothing from VOIP. There are no authorisation rejections and no other error messages.

Now, I do not know where to look further.
What can I do, if there is no information in the system log and I cannot see an error message anywhere ?