Asked by: Graham Clarke, by email
Phones such as the iPhone, that have built-in GPS usually have an anti-theft feature that allows the phone to advise of its location. But the feature needs to be enabled and the phone must be switched on. This is also only designed to allow the owner of the phone to locate it but, in theory, the police could also access the feature.
A lower-fidelity version of this is already used by the police to locate criminals and missing persons by detecting which cellular mast your phone obtains reception from. This will normally only pin-point you to within 100m but it doesn’t need a GPS receiver and it also does not need you to be actually making calls. As long as the phone is on, it will work.