"There is growing concern within the polling business about how long 
the landline telephone survey will remain a viable data collection 
tool" because of the cell-phone only population not being 
counted.  ...  Pew estimates that interviewing a cell-only respondent 
costs approximately four to five times as much as a landline respondent."

http://pewresearch.org/pubs/515/polling-cell-only-problem

www.peoplewho.org