Installing an NTE5 is very easy, first off we should point out that officially speaking you should not fit the NTE5 directly to BT's wiring(the wiring that comes from outside the building). However you are very unlikely to get into any trouble over fitting one yourself. Here is what an NTE5 consists of :-

The front lower plate comes off :-

And finally the middle plate which reveals the backbox which you may or may not need :-

If you are just fitting the NTE5 without extension wiring all you need to worry about is the back of the middle plate :-

The IDC connector has three slots but only two are used, these are marked A & B :-

Simply install the two wires that come from BT's master box(!!) into these two connections. Polarity doesn't matter(usually!) but if you have access to a multi meter then A is positive and B is negative. You should have around 50V DC across those. If upgrading from and old BT socket to an NTE5 connect 2 to the B leg and 5 to the A leg.
Now if you want to fit extensions around the house you need to look at the back of the lower front plate :-

You will see these are marked 2, 3, 4 & 5 :-

It has been proven that the 3rd wire CAN cause interference on ADSL equiped lines, for this very reason the latest version of the NTE5A has a filter on this wire. Simply connect wires 2, 5 & 3 on the NTE5 to the same connections on your extension socket(s).

