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Area code 901 serves Memphis, Tennessee, and most of its inner-ring suburbs. Geographically speaking, it is the smallest area code in the state.
Area code 901 Wikipedia
901 was one of the original area codes created under the North American Numbering Plan in 1947, and covered the entire state of Tennessee.
In a flash-cut split sometime during 1954, area code 901 was restricted to West Tennessee; while the eastern two-thirds of Tennessee—essentially, the entire state east of the Tennessee River—became area code 615.
Despite the presence of Memphis, the state's largest city, West Tennessee is not as densely populated as the state's other two Grand Divisions. As a result, 901 remained the sole area code for West Tennessee for 47 years.
However, by the turn of the century and millennium, 901 was close to exhaustion due to the Memphis area's rapid growth, as well as the proliferation of cell phones and pagers. Additionally, the Memphis LATA extends some distance into northern Mississippi, meaning several numbers on the Mississippi side of the Memphis area (part of area code 601 before 1999 and area code 662 since 1999) weren't available for use. On September 17, 2001, area code 731 was assigned to most of the old 901 territory outside of the immediate Memphis area. Currently, area code 901 serves only Shelby and Tipton counties and portions of Fayette, Lauderdale and Hardeman counties—roughly coinciding with the inner ring of the Memphis area.
901 is currently one of the few urban area codes that has not been overlaid, making Memphis one of the largest cities where seven-digit dialing is still possible. Under current projections, it will likely stay that way until 2036 at the earliest. Despite the Memphis area's rapid growth and the continued proliferation of cell phones, 901 is nowhere near exhaustion.