There's a preponderance of call centers in the area within a couple-hundred-odd mile radius of Omaha, NE, as that's supposed to be the most "neutral" accent in the Continental US. Particularly, that includes the metro areas of Des Moines, IA; Omaha and Lincoln, NE; and Sioux Falls, SD. (I'm not sure if Kansas City, {KS,MO} is also included in that set. I grew up in one of the cities with a lot of call centers, and the MO accent, at least, is pretty clearly distinguishable to me.)