NANPA is announcing a new area code for Chicago, Illinois: area code 872 will overlay both 312 and 773 area codes which cover Chicago and its surroundings (including O’Hare and Midway international airports for 773). The new area code will become active on November 7, 2009. This is the day when the 3 month permissive dialing period (started on August 7, 2009) will end for this new overlay area. Starting November 7 all calls within area codes 312, 773 and 872 must be dialed using 1+area code+local number.
See NANPA announcement + detailed map of the new 872 area code (PDF document).