The 872 area code covers the entire footprint of the City of Chicago. Right now, seven-digit dialing will work within your own area code.
But, starting this Saturday, everyone will have to dial 11 digits. That's 1, plus the area code, and then the phone number.
You may have to reprogram equipment, such as fax machines and security alarms, to dial 11 digits so they will work properly on Saturday and beyond.