sara3 and Invisible are both right. But Invisible has brought up a weakness. We are all guilty sometimes of completing forms and giving our ex-directory number. Sometimes this is required information - i.e. compulsory.
Many firms sell this information on to others - so should there be a system whereby you can indicate the telephone number is not to be passed on?
I always give an old telephone number of mine which is still not in use (I do check from time to time) on these forms - they do not need my telephone number.