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You cannot design such device without operator's license, at least in civilised countries.
Some cell phones do operate in walkie talkie mode. Consult your local cell phone company.
I believe that designing a small handheld device which can achieve this kind of range would be quite a difficulty..... The way that this type of communication is generally accomplished commercially is via long range repeaters, be they transceiver towers, or satellites in orbit.
To achieve that kind of range in the realm of hobby electronics without the use of repeaters, cell network or otherwise, would almost certainly involve directional antenna {which make the "hand held" part kind of a problem} and also relatively high levels of emitted power, which is also goes against making something easily handheld.
I agree that if there is cell phone service available in your area, that this would be the cheapest and fastest route to getting communications going. If not, you can generally set up "towers"-- IE a pole, or elevated antenna, between fixed points equipped with directional antenna to communicate with one another, and then communicate with the tower from your hand with a small, lower powered device.
You must understand however, that this would not be a trivial undertaking. It will not be a simple "circuit" which provides this without taking advantage of an already established communication network.
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