Callers will need to dial the area code and the seven-digit phone number to complete local calls in some areas of the Hudson Valley.
The change, which began Sunday, will apply to both the 845 area code that includes Dutchess, Orange, Putnam, Rockland, Sullivan and Ulster counties as well as 914 - which covers Westchester County.
The change was made to make way for the new national suicide and mental health crisis hotline number: 988.