Cellular telephones are perhaps the biggest threat to survey data that epidemiologists have confronted in years.
...Federal law requires that calls to cellphones be hand-dialed; it is illegal to use automatic dialers, which are standard tools for survey and polling firms. Furthermore, a huge fraction of "owners" of cellphone numbers are children ineligible for the health surveys. Once reached, some cellphone users are reluctant to talk at length because they have to pay for incoming calls.
The article also describes "mode effects:"
For example, when a group of people with the same age, race and education are called on a conventional phone, 25 percent say they smoke, but on a cellphone 31 percent say they do. On a land line, 38 percent say they have been tested for HIV, while on a cellphone 54 percent say they have.
These are all serious problems with few good or inexpensive alternatives.