Area Code 240-399 located in ROCKVILLE, MONTGOMERY, MD, more detail is as below. Place NPA (Numbering Plan Area) : 240 NXX (Local Number Prefix) : 399 City : ROCKVILLE County : MONTGOMERY State : MD (Maryland) NXX Use Type : Landline NXX Intro Version : 1999-10-13 FIPS (Federal Information Processing Standards) : 24031 LATA (Local Access Transport Area) : 236 Rate Center : WSNGTNZN10 ZIP Code Count : 0 ZIP Code Frequency : Second most frequently occurring ZIP Code, although percentage is unknown. ZIP Code : 20852 Other Information NPANXX : 240399 County Population : 0 Switch CLLI (Common Language Location Identifier) : WASHDCCZDS0 MSA CBSA (Core Based Statistical Area) : Washington-Arlington-Alexandria, DC-VA-MD-WV MSA CBSA Code (Core Based Statistical Area) : 47900 Time Zone : Eastern (GMT -05:00) Observes Daylight Saving Time : Yes Overlay : L Company : XO MARYLAND, LLC OCN (Operating Company Number) : 4773 Coverage Area Name : CHEVY CHASE Latitude : 38.9458 Longitude : -77.0632 ‹ previous : 240-398 next › : 240-399 Telephone Number List 240-399-0000 ~ 240-399-9999