When this comes online (I assume I can’t at the moment YO postcode); Why are we going to need a special phone? Why can’t we use an app on our mobiles, or even just a third party voip app like sipgate, if you don’t want to create your own?
You are assuming that BT are using SIP, and not a more direct connection like ISDN, which does not use the Internet. ISDN is well established within BT, and is 100% secure and has guaranteed call quality.
There is no need for BT to use the Internet for routing, as traffic can be inserted directly into the core ISP nodes.