What is what area code is 857?

Area code 857 is an overlay area code serving the greater Boston, Massachusetts area. This means it was added to an existing area code (617) to handle increased demand for phone numbers. Calls within the Boston area may be made between numbers with area codes 617 and 857 without needing to dial the area code. However, long-distance calls to and from 857 will require dialing the full area code.