By late 1996, the proliferation of cell phones and market reforms related to deregulation by the Telecommunications Act of 1996 required an additional area code for northern Missouri. On April 10, 1997, Southwestern Bell declared a Jeopardy Situation with the Missouri Public Service Commission (MPSC), which oversees telecommunications in the state, and the North American Numbering Plan Administration (NANPA).[1] On July 28, 1997, the NANPA (Bellcore) announced an area code split, in which the Kansas City metropolitan area and the St. Joseph area would retain area code 816, while the remainder of the NPA would receive the new area code 660.[2] Area code 660 comprised the rural eastern and northwestern portions of the then-current 816 area to minimize disruption to subscribers in the more densely populated urban areas.[3] The rate centers of Lexington (259) and Warrensburg (429) moved to 660, despite being generally considered a part of the Kansas City area.
On June 4, 1997, the MPSC announced the split for October 12, 1997, when a permissive dialing period would commenced, until April 19, 1998, during which long-distance calls to the 660 territory could be completed using either 816 or 660.[4]
Area code 660 is one of the most thinly populated area codes in the nation, and is therefore nowhere near exhaustion. As of September 2022, the projected exhaust date for 660 is the third quarter of 2043, which has been extended from a previous estimate of 2040.[6]