IIRC whet they did was get a DECT phone with a number of handsets and stopped using the extension wiring.
Re what Kelly's did, have you explained that to Zen?
Is the new socket an NTE5, and where is the VDSL2 filter - at the old or new socket? It is possible for this (observed) wiring to be valid - which isn't to say that in this case it is.
Thank you for your interest in this issue. To answer your questions in the order posed;
There never has been any extension wiring. A master and slave wireless phone is currently used from the Gigaset range.
Yes I did but they said that as the installation was carried out in January they could not go back to Kelly's to lodge a complaint about the installation.
Yes it is, the marking on the faceplate says "openreach MK2" and has a socket for the router to be connected to and below it a normal phone socket. The original socket in the hallway is unfiltered with no devices connected. The Openreach modem is connected to the new filtered faceplate in the living room. Again there are no fixed extensions from this faceplate. I would say that by observation it must be unusual to have two master sockets on the same exchange line with both "live".
From the Openreach modem, a CAT5 cable is connected to the router's WAN socket. There is nothing connected to the router's ADSL socket. When loop through was connected, the voice service is connected to the router's line socket with the wireless base unit connected to Line 1. All services are configured to use WAN2. The desired setup works just fine but for the ghost calls.
Now moving on. Last night the Ghost call came in unusually at 03:30, same thing two rings only. The call was unanswered because a genuine call would ring for longer. There's never been any useful information gleaned from 1471 so ignored doing that. This is getting beyond a joke while Zen seems to be wishing the issue will resolve itself with no intervention.
Zen have asked us not to loop through the router [2830vn], which we use for VoIP, and so the Gigaset base unit is now currently connected directly to the new master socket. I should state that the Zen VoIP service is outgoing only so I don't see that removing the loop through is at all relevant to the ghost calls. In any case, for the time being, the VoIP service is no longer configured in the router.
On the Ofcom front their documentation clearly states that issues of this kind can only be resolved by the telephone provider. So moaning to them is not an option.