Area code 872 is a North American Numbering Plan overlay of telephone area codes 312 and 773 in Chicago, Illinois, and entered service on November 7, 2009. Its overlay status means it is assigned only to new numbers in the area and not to existing numbers using the 312 and 773 area codes. It is the third overlay area code in the region and the tenth area code to serve northeast Illinois. Its introduction coincided with the new requirement all dialers in Chicago must dial the full 11-digit phone number (10 digits from cell phones) when placing local calls. The need for this new area code came from the existing area codes reaching exhaust status, which resulted from a combination of population growth and a large growth in cell phone usage.