I doubt that any general guidance can be given owing to the many unknowns.
How affluent or poor are the inhabitants likely to be ? Telephones only for the well off, as is the case in many poorer countries, or one per household as is the case in many countries, or several per household is is becoming the norm in some places.
What is the likely cost and availability of cellphone service ? an expensive luxury, or competitive with land lines.
Is internet service to be provided via telephone lines, or by some other system such as cable TV.
Are any busineses that require numerous phone lines likely ? a modest size call center can require hundreds of lines.
It would be well to allow for expansion as demand for communications is increasing rapidly.
