Assuming that is the only Master Socket, not one someone has added since and there is another somewhere, then gently remove the bottom half where the screws are. If there are wires from the back of the removed faceplate, and all extensions stop working when you plug a phone into them, then you don't have (what we call) star wiring.
Say there are three extensions. If each of the used terminals has three wires in, in a way that is star wiring, but it is star wiring from a junction box somewhere before the master that is problematical. From the faceplate terminals it is fine.
It would be an idea to upload a photo to somewhere of the back of that faceplate so we can check the wire colours and the terminals used. In particular, T2 and T5 are vital, T3 is redundant and can cause loss of speed, and the wire colours on T2 and T5 matter.
Any gain we can achieve on a long line like that is welcome
The indispensable man or woman passes from the scene, and what happens next is more or less the same thing as was happening before.
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