The quantity of telephone numbers depends on the population of the state. That is the reason why there might be only one area code in a whole state. South Dakota is one of such example, when there is simply no need to issue additional area codes.
Area codes on the map of South Dakota
As you can see on South Dakota area code map, there is a single phone number there. 605 is the area code which covers whole territory of the state. It means that every phone number from SD starts with these digits.
Area code map for South Dakota is painted in one color. If there were two or more area codes used in different parts of SD, it would be divided into several colored zones.
Area code of South Dakota, SD
Area codes in other states of USA
For more information about area codes in other states or in exact cities, you can follow these links:
All area codes of USA are listed here.