Have you ever filled in a form on screen, and it asked for your telephone number, and you didn't want to give it because there's no point in them calling you, but the form insists that you provide a number? There may be a simple answer if you are in the UK. Just enter the number 0333 8888 8888 (last '8' is not needed, but makes it easier to remember). If somebody rings this number they will be told that you don't want to be contacted by telephone and please use another way of contacting you.
This is a service provided by TrueCall. They don't mind