 did not serve some particular usage. That cost is the incremental cost of subscriber activity. An incremental-cost model does a poor job of reflecting the total cost of a system, but does an excellent job of highlighting the demand that directly drives up total cost. Peak-hour calls are the expensive calls for a cellular provider because it is the busy- hour call volume that determines how much equipment we need. Roughly speaking, the incremental cost of each busy-hour call is the total cost of the system divided by the to- number busy-hour calls. 1 The rest of the calls at the other times have an incre- mental cost of zero because the system is designed to have enough capacity for the peak time. The concepts of peak-time and off-peak usage in telephony go back to long before cel- lular was invented. Most cellular service in the United States has a cost structure that favors off-peak usage because the incremental cost of that usage is small. However, we do not increase our revenue by getting users to use the off-peak min- utes they have already paid for. Our goal would be to find people who would go counter to the trend and tend to use their cell phones late at night and on weekends but not much during the day. This might be hard to do. 2 What is less obvious is that some geographic areas of service are costing the wireless service provider far more than others, and we can market to potential subscribers who will tend to use their cell phones in those areas. Just as we have high incremental costs of peak-hour calling, we have high incremental costs of peak-area calling. While we may or may not be inclined to have different rates for different areas, we can encour- age growth in the areas that cost the least to serve. 1Those who have a background in economics will please forgive this oversimplification. We are ignoring the time value of money, the fact that equipment has a life cycle, or even the difference between fixed costs and continuing costs. The point we are making here is simply that the frac- tion of cal