It is Zen Internets Digital Voice. I spoke to them at length and much as though they would like it to work on something other than Fritzbox, it doesn't. Also I very much suspect that this may be based on BT's Digital Voice since BT seem to indicate ( but won't exactly admit that ) they have the same issue with their service.
What I have had to do is order the fiber from Zen and the voice from a completely different company that works with a Cisco ATA, so that I can continue using the Netgear. Thing is I can imagine a lot of folks won't do that.