Bluebell count yourself lucky that your friend is 83 and still savvy. The lady I am concerned about is nearly 90 but although savvy in many ways just cannot grasp the concept of scammers bless her.
The suggestions made by all those who have responded are all much appreciated.
Sadly the obvious ones, such as letting the call go to ansafone and then only answer if it is one of the family, are in her mind when we discuss it - but as soon as the phone rings panic/excitement/whatever sets in!
Thanks to everyone for all the suggestions - I am currently making laminated little signs to stick to her handsets!
We'll keep trying!