I will shortly be getting my master socket replaced with an NTE5 and I
am taking the opportunity to rationalise the extension wiring. I have
a short, medium and long term plan for this:

1. Initially I can get it down to two extensions - one is the link to
the burglar alarm and the other is an extension to the study upstairs
where the main telephone and the router are both plugged in. This
relies on an ADSL microfilter/splitter at the extension socket. This
arrangement has to stay in place for a while during work in the hall.

2. In the medium term, the main telephone can move down to the hall
and be plugged directly into the master socket. I want to install an
ADSL splitter faceplate, so that the extension to the study becomes a
dedicated filtered ADSL line.

3. In the longer term, I want to move the router down to the hall to
be plugged directly into the master socket. At that point, the study
will be served by ethernet lines from the router and I will be down to
a single extension from the master socket (for the burglar alarm).

To this end, I have installed cat5e cables from the hall to the study
and I intend to use these for the various purposes outlined above. It
is the initial stage that I am wondering about. The problem is that I
need both extensions to be connected to the unfiltered connection in
the master socket. But unless I use cat5e for the burglar alarm
connection (which I don't want to assume will be possible as it's out
of my control to some extent), as I understand it I can't mix cable
types in the same IDC connector. I have seen junction boxes but they
only seem to have one set of connectors coming in and one coming out.
My thought is to do something like the following:

Connect two (normal, same type) telephone cables into the unfiltered
master socket connections. One goes to the burglar alarm, the other
is just a short piece to a junction box. I then connect the cat5e to
the other side of the JB.

Does this sound reasonable? If so, a follow up question is: where do
I put the JB? I am planning to fit a flush mounted back box (as for a
standard mains socket) to take the NTE5. Will there be enough room in
there? Or do I need another single socket box next to it to house the
JB, perhaps with a blank plate on it? This would be a bit of a pain
as the JB is only needed in the short term.

In case it helps, I was planning to mount a double socket back box
next to the NTE5 anyway, with a modular faceplate for the ethernet
sockets. Maybe there would be enough room in here for the JB?

Thanks and sorry for what has turned into a long post.