Any RF signal in space around the cell phone or near a base station can be detected with a RF detector diode installed in a half-wave dipole (there are other sensors but this the simplest.
You need a Germanium diode (1N34 or 1N60) or a Schottky detector diode (Agilent HSMS series_), and you can use a thicker copper wire (1 mm or 40 mil) diameter, cut to less than one-half wave length. As the diode sits between he wire halves, the full length should be 0.47-0.50 of the wave length.
DC output from the diode can be taken via two RF chokes connected to diode ends . For cell phone wave length, 1..2GHz corresponds to 7... 15 cm half-wave length, cut a THIN copper wire (0.2...0.3mm dia) and wind a small coil on a toothpick. Cut the coil in half and pull to make a ~1-cm long coil pairs. Connect one end to the diode end (already soldered to dipole wire), do the same with the other choke on the other diode end, and this is all.
Other ends of the chokes can be connected to a 0-1 mA meter or a DVM, then hold the dipole by the diode in fingers and observe an optimum position related to signal source.
Similar way you may "harvest" a small power to feed a LED blinker, or to charge a 10-uF capacitor. Such detectors can output 0.1 to 10 mW output but usually not more as diodes saturate.