My mother is 90, she gets calls all the time that are withheld or from international sources . we tried the BT thing where witheld are barred, that was costing money and calls were still getting through, and some were trying to get her debit card number. the only answer was either to change the phone number or have an answerphone machine with voice monitoring, that way if its a friend or relative they can speak after the tone and she can hear them and pick up the phone, the dodgy ones hang up immediately. BT used to be helpful and actually monitor the lines, but now they are just out for money and don't care. She has only been in this house for 6 months, and the only way anyone could have got all her details ( some even know her age and tried a bit of flannel, like," you sound wonderful for your age mrs,###" ) was the fact that She is a crossword fanatic and is always entering competitions by post to magazines, who then sell mailing lists/phone numbers to third parties, who then sell them on to the third world etc.
percy.