"Bad news for you... it doesn't work now. That stopped working when they deployed 21CN a few years ago (putting the timescale in context, it coincided with updating the exchanges from ADSL to ADSL2+)."
It will work. 21CN for voice was effectively abandoned except for the areas where it was trialled (and GPO's finest were on the list of devices that BT had verified to work). The rest of us got put onto 21CN for broadband but the voice still comes from the same old "20C" switches.
It'll even work if you're lucky enough to have Openreach FTTP, where your phone line is VoIP and is delivered from the ONT.
YMMV if your phone services comes from someone else's kit - but I've seen reports from TalkTalk customers that their GPO 746 works just as well as it ever did.