Area code 857 is an overlay for area code 617, covering the Boston metro area and inner suburbs including Brookline, Somerville, and Cambridge. Introduced in 2001, 857 was implemented to provide additional numbering capacity for one of the most densely populated and economically active regions in New England. Ten-digit dialing is required throughout the area. While 617 remains the legacy code, 857 is widely used for mobile numbers, startups, and new residents. It reflects the region’s ongoing expansion in education, healthcare, and technology. Despite being newer, 857 is now an integral part of Boston’s telecommunications landscape.