Hmm, BT can use VoIP at the exchange end. We had a DSLAM that did broadband and VoIP termination for the POTS line five years ago. They were on the same card and dispensed with the need for an exchange to offer POTS. I expect they are using the same design for 21CN as we proposed.
We also had cards that did traditional trunk lines that just replaced the concentrators and ran 2M back to the traditional switch.