Thank you those who replied. I have now checked the wiring & found no visible fault. The cable looks correct: solid conductors; colour- coded as on WPP website; pins 2 , 3 , 5 connected in correct colours at both ends; 4 not connected. I cannot tell if the cable is twisted without stripping some and there is no spare length. Massive resistance between conductors ( > 10 megohm). Unplugged master socket, shorted one end, there-and-back resistance about 3 ohm. Sometime I hope to lift a floorboard & see how close it runs to the mains cable...
In the meantime the trick of using a master socket at the extension and connecting to 2 and 5 only may work. Hum is often picked up in the unbalanced 3rd ringing line.
Interesting thought. Never even considered this - I started doing this on a PABX at work and it seemed fine. At our last house I had the BT master socket and then seven "PABX masters" (i.e. without the gas discharge tube - figured the wiring would blow before the tube if struck by lightning :-). There was a phone on BT's socket and five phones and a modem in the others (one unused socket). Never seemed to cause problems, though I don't really know how far we were from the exchange.
As someone else has said, you may disagree with the regulations (and I can understand why - the system is unlikely to be unsafe in practice) but they do state that all cables run together should be insulated for the highest voltage present:
528-01-02 "A band I circuit [e.g. telephone] shall not be contained in the same wiring system as band II [e.g. mains] voltage circuits unless one of the following methods is adopted:
(i) every cable is insulated for the highest voltage present (ii) [discusses multicore cable] (iii) [multicore again] (iv) the cables are insulated for their system voltage and installed in a separate compartment of a cable ducting or trunking system (v) [trays and ladders] (vi) a separate conduit, trunking or ducting system is employed.
[...]
For telecommunication circuits, data transfer circuits and similar, consideration shall be given to electrical interference, both electromagnetic and electrostatic. See BS EN 50081 and BS EN 50082."
Comments in [] by me.
In the OSG this is tabulated in tables 7.3A and 7.3B, section 7.3.3 page
As you have found, there is also a very real risk of interference. Standard mains twin and earth isn't a particularly noisy cable, as equal and opposite currents flow in close proximity, but get *too* close and you'll hear it. (Just thought - if it's a ring circuit it's worth checking that both L and N are continuous right around the ring. If one of them is broken this will lead to unbalanced currents in the loop and will greatly increase the potential for interference). Proper telephone cable should have a twist in each pair which also helps reduce interference, but it isn't perfect.
If you *must* wire this closely, instead of standard telephone cable, choose something like Cat 5 cable which is usually (though not neccessarily always) insulated sufficiently (some brands are marked
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