NPA-NXX - 312-399

This is the 312-399 subcode (NPA-NXX) prefix page. Follow a prefix to find the number you are looking for. We divide our pages up to make them more user friendly to navigate!

1. Area Codes
2. 312
3. 312-399

The 312-399 subcode mainly covers Chicago in Cook county. But other numbers maybe included as well.

We have a database of information derived from the public phone records of Illinois state. Many of the numbers in this subcode are Miscellaneous registered to Sprint. We hear they employ a large number of people in the Illinois office.
Recently there have been an increase in reported calls, we've had over 5194675 comments across our systems for the 312-399 subcode.