From Ald. Gene Schulter's weekly email newsletter:
Effective Saturday, November 7, 2009, all local calls will have to follow new dialing procedures; this means that all local calls in the (312 or 773) would be required to dial 1+ the area code+ telephone number. If this new procedure is not used, your call will not be completed and a recording will instruct you to hang up and dial again.
This change was brought about by the addition of a new are code of 872 to the same area served by both (312 & 773) due to the increasing demand for telephone numbers. Since more than one area code will now serve the same geographic region, 1+ the area code will then have to be used. This could also mean that beginning November 7, 2009, new telephone lines or services may be assigned numbers with the new 872 area code.