Apparently you risk being charged if your fault on your line turns out to be the socket. Apparently it's your socket, not theirs, even though supplied by openreach, fitted by them, and even though it's faulty due to lightning running down their wires into the socket that's even got their name on it
As you say if something on the house side of the socket causes the problem you could be charged but if you plug a phone directly into the test socket you can find out if its the line to the house or something you or a previous owner has plugged or wired in that is causing it, i only know this because we had problems with noise on the line and fluctuating speeds and were told to try the test socket IIRC thre was still noise on the line and when he visited the engineer found damp in the socket and he swapped it for a newer version like the one below.
Your Master Socket and How To Avoid A Large Bill
As a telephone engineer, I’ve met a few customers that have ended up with a big bill all because of their master socket, and the use of it.
There’s no great mystery about that box on the wall where your phone line comes in to your property. However, you master socket
is the specific “boundary point” in your home where your telephone line provider’s responsibility ends, and yours begins.
So, the bit that’s fixed to the wall of your master socket goes to the outside world, and to the national phone network. Any faults with that are the responsibility of Openreach (the former BT engineering division who will come to your house on behalf of your provider). In the front of this socket is a faceplate that plugs into the wall-mounted bit and is secured with two screws.This brings your various extensions from around the house to one point and connects them onto the line.
Now, if you have a dead line, or some kind of noise, interference, or any other kind of problem, you need to determine if this is your fault (your wiring), or theirs. It really is simple. Take off the faceplate, and plug in a known working telephone straight into the socket that is behind it. Your wiring is then disconnected from the network, and can cause no mischief.
If the problem goes away, then the fault lies with your wiring or devices.
If it does not, then the problem lies outside your house and is not your responsibility. You need to report it to your provider, who will arrange for their designated engineer (usually working for Openreach) to call if necessary. Your provider (Talk Talk, Orange, Plusnet, etc, etc) may ask you to repeat the test, that is plug the phone into the wall-mounted socket again. One of the advatages of this is, should there be line noise, the customer service person at the other end will here it, too, and might actually believe that the “crackle” is not just in your head!
But here comes the (potentially) costly bit; if an engineer from Openreach is sent to your premises, and a fault is discovered on your extension cabling, not theirs, they have to charge for their time and resources. This can be from £132.00 upwards. Ouch.
So… you need to be absolutely sure, by use of your master socket, that the problem is not with your wiring, sockets, and devices before getting them to come to you..
https://telecomgreen.co.uk/home-phone-repair/your-master-socket-and-how-to-avoid-a-large-bill/
