Since I fitted a set of phones with the TruCall call filter system, the
nuisance calls have almost come to an end. It has logged not a single
call in over 7 days.

I don't really understand why - most of these callers just ring and
play a recording anyway, so the callers would not even be aware their
calls have been blocked. Which rather suggests that something has been
done nationally to put a stop to these callers.