It sounds very much like someone has just botched things up, rather than doing it properly. The whole lot probably needs to be ripped out and done properly. Are you using the line for Internet?
You incoming line should go straight to a master socket, then from there, it can go to any additional sockets you might need - idea is that an engineer (or you) can unplug all of the slave sockets and wiring from the line, to be able to just check the line alone easily. Modern master sockets also include a broadband filter, so a modem can be plugged straight in without needing a separate filter.
There are two versions of fibre. One has fibre to your door, the second uses copper from the box in the road.
We still don't know for certain what you have at the moment. Could you manage some photos showing the actual internal connections, including the inside of the grey box?
Many people these days use wireless phones, so much less need for more than one socket. My system has a master, with the fibre over copper modem plugged straight into that, then around 8 slave sockets around the house, which I installed in the pre-wireless days, along with LAN sockets and TV antennas. Only one of the slaves is used, that to plug a wireless base station in, then I have four wireless phone scattered around the house, in chargers (BT-8500).
The BT-8500 includes a built in nuisance caller blocker, which completely prevents them getting through - it logged 7 attempts, in the past 7 days.