Without being familiar with kabelbw, but the hostname itself sounds a bit unlikely to be the official DNS name for an ISP's SIP services (it might very well be one of the servers behind it, but unlikely to be the official DNS entry).
Given that kabelbw is pushing dslite, only offering IPv6 for their own services might not be impossible though.
this is kabelbw. Their organization and support processes are totally chaotic, messy and crap.
I have the TC4400 activated, up and running and very happy I made it that far.
There are no admins to contact, I have doubts that there is any chance to get this cleared on their side.
It was just intuitive that I wanted to somehow do a forwarding / setup of a proxy to let the internal ipv4 client talk to the external ipv6 something. I thought that this could be done on OpenWrt since there is stuff like NAT64.
NAT64 can make the messages flow but it can't make the messages meaningful it's like if you send me letters in Chinese and someone translates the address for you and puts it in an envelope that American postal service can read, I can get your letters but I can't understand them.
In particular, how do you think your SPA112 will like it when it gets an INVITE message saying to connect back on 2002:2010:aaff:1100::ba2b port 23000 ?