To use the "Find my iPhone" app, you would have had to have that setup prior to loosing the phone so that you can track it.
JorgeM
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All cell phones are trackable, using their unique imei number. That is registered with all cell towers in range of the device when it is turned on. That is what the "Find my iPhone" app uses to track your phone if it is lost or stolen. Even if the SIM is changed, the IMEI number is hard-wired into the phone. I don't think it can be changed.
rubberman
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