A television news editor would like to know how local registered voters would respond to the question, " Are you in favor of the school bond measure that will be voted on in upcoming election?" A television survey is conducted during a break in the evening news by listening two telephone numbers side by side on the screen, one for viewers to call if they approve of the bond measure, and the other to call if they disapprove. This survey method could produce biased results for a number of reasons. Which one of the following is the most obvious reason?