It is illegal for civilians to tap someone’s cellphone. Law officials can tap cellphones legally only under certain conditions, namely, either with a court order or under the one-party consent provision.
To receive a court order to tap a cellphone, law enforcement officials must have probable cause. Wiretap orders are time-sensitive, usually permitting legal wiretapping for up to 30 days. Law enforcement officials must notify the tapped party once the court order expires and the wiretapping is complete.
Under the one-party consent provision, federal law enforcement agents and law officials in California are able to tap cellphones with the consent of one person in the conversation. For example, federal officials do not need a court order or additional paperwork to tap the cellphone of a government informant.