Well, I have finally got my old analogue wiring connected into digital voice such that handsets ring in tandem and they can be picked up in the different rooms and join/take over a call. It was both easy and complicated so I provide notes for anyone wishing to do the same.
Mine’s is an old installation put in between 2007 and 2017 (based on the vdsl master socket type in place). I have FTTC meaning 2 wires (‘copper’ ) come into the house to a small junction box, which is wired through to an old BT master phone socket in the hall, the two wires (via jelly crimps) then being cabled to a wire to an NTEC5 master socket (type A) at the back of the house. This same cable takes three wires back to the hall master socket where they are idc crimped into the old phone master socket. This provides the wiring to my analogue phone extension points.
After DV switchover the telephone socket on the vdsl master socket no longer provided telephone service, and my old extension sockets were also dead.
To reinstate things, I disconnected the three telephone wires in the vdsl master socket (which had been inserted into a 3 pin idc block on the face of the cover) and jelly crimped them to three wires in a length of phone extension wire. The other end of this extension wire was connected in the matching colour /post number sequence as had been in the vdsl terminal block. This was to a double telephone socket wall plate with a back box. In effect I moved the three telephone wires from the vdsl master socket to an extension box, thus fully isolating my internal telephone wiring from any vdsl master socket connection.
I didn’t know whether to use a telephone slave or a telephone master wall plate (which has extra capacitors and resistors in it) for the extension box. So I tried both and found it made no difference.
I then used a double bt-phone plug-ended cable to connect the green phone socket on the back of the smart hub 2 to my extension box. This was a straight through cable sourced from *bay. I didn’t know whether it should be a crossover cable or straight through cable. I picked the latter and don’t know what difference it would have made.
I then plugged my analogue desk phone into the other extension box socket.
Success! I now had a dial tone on all 3 of my analogue phones and they rang in tandem for an incoming call. However there was a noticeable amount of noise on the phone line. I checked all my cable connections without resolving this so swapped my desk phone back to the direct hub connection (losing my other phones of course).
I read that improvements in filtering were introduced by later vdsl master sockets than mine, so decided to try a vdsl microfilter sold by a specialist telephony bits supplier. When it came I plugged it into the green telephone socket of my smart hub 2 and the bt plug to bt plug cable from there to my extension box. It didn't solve the noise problem however. At this point I thought the problem must be in the vdsl master socket, even though the telephone wiring was now separated. So I resolved to try the vdsl microfilter in the test connector of the vdsl master socket but before I could do so my broadband service download speed crashed from around 30Mbs to 2Mbs. I called the service line to see if there was a service outage, the BT Guide checked settings and made some adjustments (didn’t say what) and offered to send an Openreach engineer around to check my installation. Two days later he came, installed a new vdsl master socket (mark 4 type) without me asking and replicated my extension box for the telephone wiring. Thereafter all has been fine, same broadband speed as before and analogue telephones all working with no line noise. Hurrah!
I want to give my thanks to the contributors to this thread @licquorice , @imjolly , @jac_95 , to the Posters in other threads who provided the method, to the Openreach engineer and to the BT Guides, all of whom have helped me to the solution.
I recommend if you are in a similar situation, your also reach out for the help on offer.
