Area codes 0-99 serve the metropolitan areas of Mexico. The Country code nationally is +52.
Area code 33 serves the Metropolitan area of Guadalajara, Jalisco, area code 55 serves the Metropolitan area of Mexico City (Mexico State and the Federal District) and area code 81 serves the Metropolitan area of Monterrey, Nuevo León.
(For other areas, see Area codes in Mexico by code).
References
Area codes in Mexico by code (0-99) Wikipedia(Text) CC BY-SA