New details of the Paris attacks carried out last November reveal that it was the consistent use of prepaid burner phones, not encryption, that helped keep the terrorists off the radar of the intelligence services.
As an article in The New York Times reports: "the three teams in Paris were comparatively disciplined. They used only new phones that they would then discard, including several activated minutes before the attacks, or phones seized from their victims."
The article goes on to give more details of how some phones were used only very briefly in the hours leading up to the attacks. For example: "Security camera footage showed Bilal Hadfi, the youngest of the assailants, as he paced outside the stadium, talking on a cellphone. The phone was activated less than an hour before he detonated his vest." The information come from a 55-page report compiled by the French antiterrorism police for France’s Interior Ministry.