Re: Wither the 21CN?
BT (largely but not exclusively Openreach) had two largely independent flavours of 21CN, though the 21CN publicity often muxed them ip.
One flavour was for mass market broadband via xDSL, which was (sort of) planned, rolled out, and by now is mostly superseded.
The other flavour was "21CN Voice" (see e.g. https://www.theregister.co.uk/2006/11/28/21cn_online/ which relates to BT's first public implementations of 21CN).
See also e.g.
21CN voice rollout was "suspended" shortly thereafter, and in due course, abandoned.
Hopefully the people who f***ed up the 21CN Voice design will have moved on by now.
So this most recent announcement is presumably 21CN Voice V2.0?