A cell phone is a radio!
All you need is a miniture external Yagi Antenna (this is a directional antenna, which could give you an additional 6-12 dB gain, this means you would have to point the antenna in the direction of the nearest cell tower) and a way of connecting the antenna using 50 ohm coxial cable to the antenna port on your cell phone. i.e A BNC or TNC phone adapter, thats of course if your cell phone manufacturer makes such a thing.

If you were to use a omnidirectional antenna you would need an external microwave amplifier (heavier solution also requiring additional battery power) to get a similar gain.

http://www.alternativewireless.com/cellu...-dual-band.html