SAN FRANCISCO —Consent to search a driver’s cell phone does not authorize a border patrol agent to accept incoming calls and impersonate the owner, the Ninth Circuit ruled Thursday in the latest decision testing the boundaries of privacy in mobile devices.

“An individual who gives consent to the search of his phone does not, without more, give consent to his impersonation by a government agent,” Judge Stephen Reinhardt wrote for a unanimous panel in United States v. Lopez-Cruz.