Scott Keeter explores the issue at the Pew Research Center.

Twenty years ago the survey research profession — having grown comfortable with telephone interviewing as an alternative to personal interviewing for conducting surveys — worried mostly about the roughly 7% of U.S. households that could not be interviewed because they had no telephone. Today our concern is somewhat different, and potentially more serious. According to government statistics released last month, nearly 13% of U.S. households (12.8%) cannot now be reached by the typical telephone survey because they have only a cell phone and no landline telephone.