Area Code 240-558 located in ROCKVILLE, MONTGOMERY, MD, more detail is as below. Place NPA (Numbering Plan Area) : 240 NXX (Local Number Prefix) : 558 City : ROCKVILLE County : MONTGOMERY State : MD (Maryland) NXX Use Type : Landline NXX Intro Version : 2001-06-30 FIPS (Federal Information Processing Standards) : 24031 LATA (Local Access Transport Area) : 236 Rate Center : WSNGTNZN11 ZIP Code Count : 0 ZIP Code Frequency : Second most frequently occurring ZIP Code, although percentage is unknown. ZIP Code : 20850 Other Information NPANXX : 240558 County Population : 0 Switch CLLI (Common Language Location Identifier) : HRNDVA13DS0 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 : CAVALIER TELEPHONE MID-ATLANTIC, LLC - MD OCN (Operating Company Number) : 3910 Coverage Area Name : KENSINGTON Latitude : 39.0260 Longitude : -77.0778 ‹ previous : 240-556 next › : 240-558 Telephone Number List 240-558-0000 ~ 240-558-9999