Two senators have introduced legislation requiring consumers to produce identification before buying such phones.
That's rattled some civil liberties groups, who say the bill may threaten privacy rights.
But because they can be purchased anonymously, they're a favored tool of drug dealers and other criminals looking to cover their tracks.
Prepaid phones also have been linked to suspected terror activity.
Authorities say accused Times Square bomb plotter Faisal Shahzad used one to purchase a car in which to hide a bomb.
