Hi, no these arent for adding extensions.
they go on your BT NTE5 Master Socket
you remove the lower half, and replace with the plate, the one I listed is the latest one from BT Openreach and has the built in RF bell wire filter which improves your broadband speed slightly, but apart from that it basically replaces the ADSL filters you normally have to plug into every phone socket in your house, as the upgrade faceplate filters the whole house telephone wiring. Also because it has the ADSL and Telephone socket built into it, you can plug your modem straight into it.
Idea is, you loop out of the lower half of the BT NTE5 Master Socket, to your Slave sockets in your house.
you cant loop out of an NTE5 master socket from the front lower half though into another NTE5 master socket as thats for looping out for slave sockets, if you need several of these in your house you need to loop out the top half of the NTE5 socket.. but touching the top half is BTs property, BT only allow you to touch the bottom half, hence it being removable... people dont really take any notice lol but just be aware of it.
there is a voltage yes,
50v dc, it jumps to 75ac on incming call according to BT openreach engineer.
but the amperage is so low you can touch the bare wires and dont feel a thing.