On Mon, 5 Oct 2020 11:21:49 +0100, TimW wrote:

decorating a spare bedroom upstairs I found the old telephone extension
socket behind the bed and cheerfully pulled it off the wall snipping the
cable where it came through the window frame saying 'no need for that
anymore!'. since then several people have said they have had difficulty
ringing us up (although there is a dialling tone and broadband still).

Is this coincidence?

I doubt it... What do callers get? Ringing tone but no answer, brief
ringing tone that cuts to silence, immediate engaged tone?

Is there a proper way to terminate an unwanted telephone cable, and what
is it?

In the short term ensure that none of the wires are shorted to any
other. Longer term fully remove the now redundant extension wiring
right back to where it splits from other wiring. The unterminated
stub of cable may knock a Mbps off your broadband speed. Wether that
is a problem depends on what speed you get, 50+ Mbps probably not an
issue, down at 5 Mbps every kbps counts...

You could just follow the cable and discconect from the main wiring
but that isn't "neat and tidy" in my book. B-)

--
Cheers
Dave.