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.)
I always figured it was a timezone thing that they eneded up there. They'd end up at most 2 hours difference from all major time zones, and average about 1.33 hours across them all. That makes it much easier than getting people to work by 5am in california to be able to handle the east coast, or working until 8pm on the east coast to handle california.
Having lived in KC for 10 years, I never noticed an accent at the time. But then after living in the Northeast for a while, the KC accent seemed to have a very distinct Southern drawl. Not as much as e.g. Texas, but there nonetheless.
