Yeah it seems the CVLANs are required, i.e. added by the CMSAN to differentiate h.248 traffic such that it goes to the correct call server and Broadband traffic to the correct bRAS:
https://imgur.com/a/PSfgec8. I.e. If PSTN traffic appears on X interface, then send the mac/ip/sctp/h.248 frame constructed to Y IP and Z MAC addr of the call server and add W CVLAN (or let it be automatically added through inter-vlan settings for sending to that IP). I don't really know. Potentially, and rather irrelevantly, both traffic could be routed through the CP bRAS(*), a few diagrams suggest this:
https://imgur.com/a/XFomXm8, others do not:
https://imgur.com/a/GhT2uqV. I suppose all customers connected to a specific CP bRAS and the bRAS itself would all be on the same CVLAN and it would function a bit like the CableLink system in place for NGA.
Update: The diagram showing a softswitch aside the bRAS would represent the BT Wholesale softswitch which I think is already or is going to be used to replace the BT Tandem network (LLU SMPF). The one after the bRAS would represent an ISPs softswitch for an ISP that provides voice I'd imagine.