Our solution is an answerphone, combined with caller display. If no number is shown, or we don't recognise it, we leave it to the answerphone. If they don't, or can't, leave a message we ignore it. Some phones (our Panasonic included) have a facility to add numbers to be blocked, which we have used for a particular series of fake "charity collectors". Some phone service providers, such as Tesco, also provide a blocking facility, at a charge. For both these options, of course, you need to know the number which called you.