The only legal way to obtain and use a scanner for cellphones is to receive a valid warrant as a law enforcement officer. Using a scanner to listen to cellphones under any other circumstances is a federal crime in the United States. It is punishable by fines, prison time and civil suits.
It is legal to scan for unencrypted broadcast frequencies in most jurisdictions, including police and fire radio, weather radio and amateur radio frequencies. One easy and inexpensive way to obtain these scanners is to use a smartphone app. Apps such as Scanner Radio allow users to browse broadcasts by source, area and type. Some apps offer only live access, while others archive broadcasts for future listening.
Although scanning for unencrypted frequencies is legal under federal law, states are permitted to pass more restrictive laws. New York and Florida prohibit scanners in vehicles except by licensed radio operators or individuals who legitimately use scanners as part of their jobs as of 2015. Michigan prohibits the ownership or operation of a scanner for five years after a felony conviction.
Using a police scanner or other legal scanning equipment to aid in the commission of a crime carries additional criminal penalties in some jurisdictions.