it depends what you are trying to do. If it is for inter-office use, then analog gateway should be sufficient, between 2 to 8 lines. if you are needing a larger number of lines, then a digital gateway is a better choice.
Analog lines have dial tone that you could hear with a normal phone.like the one at home.
Cost from 5 dollars a month to 40-50 dollars a month depending on the country.
if you plug a phone on the oulet and hear dial tone ,it is analog.
Digital: Either a T1 or E1. Cost a minimun of 300-400 dollars a month on the cheapest places,up to $2000 or $4000 dollars a month on the third world countries and some first world places too.
Need special equipment to make calls, like a PBX ot a good instrument to test the lines.
You could have even big cities where this service is not avaialble in some countries.