What can be done about excessive robocalls and spam calls?

My mother has had the same telephone number for over fifty years.

In recent months, there have been as many as 30 robocalls a day. About a year ago, there were maybe 8–15 per day. Even that is far too many. We never answer unknown calls.

I’ve blocked some of the more persistent ones, but it still rings once and the caller ID says ‘Caller Blocked’—but we still get the harassing ring. And there are still 20–30 a day. Her telephone has literally become a tool of harassment.

Some numbers are spoofed. Do Not Call lists do not work.

Is there any way to REALLY block calls, or is there some device that can detect probable unsolicited calls?

The ringing phone is a constant, harassing problem. We are getting quite desperate.