This is how I solved a similar problem to the one you are facing.
My changeover happened today. I did not have to deal with any bare wires, or do
any soldering.
Our VM hub is in my study on the first floor, connected via the VM ethernet cable
connected to a socket on the external wall, linked to an external cable going down
to the VM junction box downstairs, outside the house.
Internet devices around the house are connected to the hub via ethernet cable (one PC downstairs) or
powerline devices using the domestic electrical circuit or using wifi -- e.g. mobile phones.
The old VM phone cable came in through a downstairs wall to a socket in the
hall, to which our phone base (including recording machine) was connected via a
short standard phone cable ending in a small plug -- part of the phone base.
In preparation for the changeover, I installed a long phone cable (made of two
cables, joined in the middle because one cable was not long enough). The new
long connecting cable has a standard phone socket at one end downstairs near the
phone-base. The other end of the connecting cable, upstairs, has a standard
phone plug, waiting until today to be connected to the hub.
That plug now goes into the socket part of the short device received from VM
several weeks ago which has a plug at one end, that goes into the back of the VM
hub and a socket at the other end, for the cable from the phone.
Now that the changeover has occurred, I've plugged in the plug end of the long
cable from downstairs (i.e. plugged it into the socket on the new device at the
back of the VM hub), and unplugged the phone base cable from the old socket
(with external connection) and plugged it into the new phone socket at the
bottom end of the new connecting cable.
The phone worked immediately with its new connection. The hardest bit was using
a lot of cable clips on a vertical post in the stairwell, and along skirting
boards and the edge of the carpet on the upstairs landing, using small very
fiddly cable clips to keep the new long connecting cable (fairly) tidy.
In retrospect, I think the original package from VM should have included a diagram
showing a new connecting phone cable between between old phone (or phone base) and new
device at the back of the hub, for users who could not simply unplug their phone
cable from the existing phone socket, and plug it directly into the new device
at the back of the hub. I am amazed that VM people just assumed that all
customers would already have their VM hub close to their existing phone socket.
