Hero.co.nz look like they have cheap DDI lines.
Thinking about playing and setting one of these up on my Fritzbox to see how it goes.

With 1st month free to play (and then $2.30 a month) might be an option to allow to finally convince my wife to let me ditch the phone line and still allow local incoming calls.

Hmmmm - They have even got a Fritzbox setup guide :) https://faq.hero.co.nz/index.php?action=artikel&cat=18&id=105&artlang=en-us

SteveIdol: This implys it works but I cant get it to play ball...
https://www.geekzone.co.nz/forums.asp?forumid=39&topicid=208634

Some of them do support analog in on the 8 pin plug for DSL on another pair, I briefly tried at one time to run it to a cellular terminal for voice to the dect handsets but gave up. Also when bringing things in over analog you open yourself up to things like echo, poor calling performance, missed calls because the analog interface eats the first ring or 2 while doing caller ID etc.

Yes, the centre pins on the RJ11 would typically go to centre pins 5,4 on RJ45. RJ45 end needs to be modified to 3,6.

https://en.avm.de/service/fritzbox/fritzbox-7490/knowledge-base/publication/show/56_Pin-assignment-of-cables-and-ports/