Nuisance phonecalls

My mother is in her 90's and in very good order. But I am very concerned at the amount of 'cold calls' she receives every day. I fear she will have a fall one of these days as a result of trying to catch a call. I have tried some years ago to 'register' her number as not to be called but feel the need to revisit this. Can anyone advise as to the best way to do this?
Also I have experienced a device when calling a person in America that said my number was not recognised and would not be put through but I had the option to leave a message. This seems a useful system but I have not seen in UK