Pinging a cell phone traditionally means locating the phone by identifying the cell tower of the last signal the phone received. These days that's not the only way to ping a lost phone.
Step 1
The easiest and most "modern" way to locate a lost phone is via the software that comes with most smartphones -- either Find My Phone on Apple's iOS or Google's Location History.
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Step 2
There are many apps which track the location of smartphones. You can use Find My Friends for iOS, for example, or WhereAreYou on any smartphone.
Step 3
The traditional approach is to request your cell phone provider to ping to cell tower and triangulate the phone's location. This is available to government and law enforcement, but generally not something ordinary citizens can do.
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