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it depends on how powerful the radios are on the tower. cell phone towers use special antennas to divide an area into slices which will have several antennas that only cover, say, 15 degrees or so. the further you are away from the antenna the wider the sector is, so they are run at a lower power to cover higher population dense areas.
so to build up the cellphone network, you need to build more towers in population dense areas.