What is random digit dialing and why are most polls conducted this way?

Random digit dialing selects a series of numbers which make up a telephone number, and calls that number. These telephone polls are conducted on massive scales because they require little manpower and are very convenient for the interviewer in order to try and discover the public opinions on certain issues. They are completely randomized, but biased in that a telephone poll is a voluntary response sample, so subjects select themselves once called, and will often not respond unless they have a very strong, often negative opinion