I am in the UK and want to make several phone extensions.

QUESTION: I would like to know I this will increase the level of hum.

ISTR UK phones have a transformer and some other components to
neutralise hum but would that be good enough to prevent hum from a messy
setup like mine? Some details are below.

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In my situation the phone extension wires and the mains wires will run
close to one other.

There will be about four or five additional extension phone sockets.

And in some phone sockets there will be a loose extension lead of approx
3 metres which will be almost ontop of curled mains flex