New York and San Francisco announced major reductions in the number of stolen mobile phones since "kill switches" were added to phones sold in these cities.

A "kill switch" is an anti-theft tool that protects the owner of a stolen mobile phone by remotely erasing personal information such as contacts and emails from the stolen phone. It also makes the mobile phone inoperable to unauthorized users.

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It allows an owner of a stolen phone to send a command that blocks it from being accessed or reset until an ID and passcode are entered.

In New York and San Francisco, kill switches installed in iPhones and Android handsets have been a major deterrent to criminals. New York reported a 25 percent fall in iPhone robberies, and an overall 16% decrease in mobile phone robberies.

San Francisco reported a 40 percent drop in iPhone robberies, and an overall 27 percent fall in mobile phone robberies. These latest statistics for both cities come from a two-year period from the start of 2013 to the end of 2014:

Apple's kill switch is the default setting on new iPhones running iOS 8 so users don't have to switch it on. Google said in October 2014 its Android Lollipop operating system will introduce an opt-in kill switch feature called Factory Reset Protection.

Samsung has a kill switch it calls a Reactivation Lock offered on some of its Galaxy S5 smartphones. The kill switch, however, must be turned on manually. Samsung was the first Android manufacturer to install a kill switch

Microsoft has promised to install a kill switch into its smartphone version of Windows 10.

New York, San Francisco and London together formed the Secure Our Smartphones (SOS) initiative in 2013 to force smartphone manufacturers and mobile networks to do more to combat smartphone thefts. The SOS initiative calls for kill switches to be automatically activated on every mobile device.