Dave Plowman (News) brought next idea :
Just the one base station? I have several. It obviously stops the one
plugged into it from ringing.

Just to make it clear - 1x base station plugged into the phone line,
with three phones each with their own drop in charger base, plus one
old none wireless phone with its ringer turned off - for emergency use
during a power cut.

The latter sits in a kitchen drawer, plugged into a socket, which is
hidden behind the unit.