In 1947, Bell instituted area codes for states and provinces in the United States and Canada under a system called the North American Numbering Plan. The plan created 86 area codes, including 505 for the entire state of New Mexico.
Area code 505 covered the entire state of New Mexico from 1947 until 2007, when regulators split the state between two area codes. A northwest section of the state kept the original area code.
The decision on how to split New Mexico between area codes sparked a political battle over which part of the state would keep the original area code and which would switch to a new area code, 575. In total, 43 rate centers remained under the 505 area code and 120 received the new area code.
Cities that stayed in the 505 area code after the split include Farmington, Los Alamos, Aztec, Santa Fe and Albuquerque. Southern and eastern portions of the state, including cities such as Alamogordo, Carlsbad, Clovis, Deming, Hobbs, Las Cruces, Penasco, Roswell, Raton, Silver City and Taos, began using 575.
