Kezo, they’ve left the old BT box with the alarm wire in place, it’s identical to the one on the link you sent. If I buy that kit is it just a matter of plugging it into the phone socket on the modem then into the old wall box, if so that’s brilliant cheers.
Yes it plugs into one of the phone sockets on the router/modem, You then remove the existing BT faceplate and replace it with the modified one in the kit (I would disconect & tape BT’s incoming wires incase it still has 50vdc on them)
If your confident you can modify your existing BT socket if you wish and just but a RJ11 to BT socket lead (or make your own), as it would work out cheaper. Let me know 🙂