Here is a list of radio interop regions:
Northeast:
Knox
Cedar
Dixon
Wayne
Dakota
Thurston
Region 11:
Antelope
Pierce
Madison
Stanton
Region 5/6:
Cuming
Burt
Dodge
Saunders
UASI:
Washington
Douglas
Sarpy
ECCA:
Boone
Platte
Colfax
Howard
Nance
Merrick
Polk
Butler
Southeast:
Cass
Otoe
Nemaha
Johnson
Pawnee
Richardson
Gage
Lancaster
Seward
York
Fillmore
Saline
Thayer
Jefferson
CNRI 1:
Lincoln
Dawson
Frontier
Gosper
Phelps
Harlan
Kearney
Buffalo
Hall
Adams
Hamilton
Clay
Franklin
Webster
Nuckolls
CNRI 2:
Perkins
Chase
Hayes
Dundy
Hitchcock
Red Willow
Furnas
Sandhills:
Deuel
Keith
Arthur
Mcpherson
Logan
Custer
Valley
Sherman
Greeley
Wheeler
Garfield
Loup
Blaine
Thomas
Hooker
Grant
Cherry
Brown
Rock
Holt
Keya Paha
Boyd
Region 21:
Kimball
Cheyenne
Morrill
Garden
Scotts Bluff:
Scotts Bluff
Banner
Region 23:
Sioux
Dawes
Box Butte
Sheridan