If you can get hold of capacitors that keep a phone signal alive for a day i will buy ten of them.
Seriously people. All they do is a multilateration of the radio signal.
Some wikipedia info:
To locate the phone, it must emit at least the roaming signal to contact the next nearby antenna tower, but the process does not require an active call. GSM localisation is then done by multilateration based on the signal strength to nearby antenna masts.