On Monday, 5 October 2020 11:21:54 UTC+1, 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? Is there a proper way to terminate an unwanted
telephone cable, and what is it?

If you have caused an intermittent short that could cause callers to receive the engaged tone, or if you have shorted the bell circuit your phones won't ring on incoming calls.

best would be to disconnect the unwanted cable at the 'supply' end.

Or use a small terminal box.

Owain