I don't know about some of South Dakota, but my wife spent her first eighteen years in what is now 605-637 ... which has no more than about three hundred active numbers in that CO, which leaves 9700 unused. (My first trip up there was in 1990, and the local telco had gone to ESS before then ... all local calls had to be dialed as seven digits.
Interestingly, there's an adjacent CO just across the state line in North Dakota (701-634, IIRC) which is run through the 605-637 (NWEFSD) switch and can be called by dialing 701 plus the local number (but if you dial with 1-701, it's treated as long distance), and has even fewer active numbers in its block of 10,000. The Kentucky coal town I was born in (now primarily 606-848, although 606-238 and 606-733 are in use now) didn't go to ESS until some time after I got my degree in the late '70s, and I was used to going up there in the summer and being able to call anywhere in the two towns it served by dialing the last four numbers. (BNLYKY, if anyone's curious). Anything outside of that, and the neighboring CO in Cumberland, was dialed with 1+.
Patrick "The Chief Instigator" Humphrey ( snipped-for-privacy@io.com) Houston, Texas chiefinstigator.us.tt/aeros.php (TCI's 2005-06 Houston Aeros) LAST GAME: Houston 5, Iowa 3 (April 15) NEXT GAME: Friday, April 21 vs. Peoria, 7:35 (Game 1)