Prefix 312-221 — Chicago Zone 01, IL

NANPA CO code assignment, current as of July 28, 2026.

The 312-221 prefix belongs to area code 312 and is anchored to the Chicago Zone 01 rate center (IL). In NANP terms, the prefix — or NXX — determines the rate center a number is billed against.

Chicago Zone 01 is part of the Chicago-Naperville-Elgin, IL-IN Metropolitan Statistical Area. For routing purposes it sits in LATA 358. Numbers here keep Central time.

NANPA's central-office code records list T-MOBILE USA, INC. (OCN 6529) as the code holder, assigned June 17, 2011. Because 312-221 participates in thousands-block pooling, individual 1,000-number blocks may belong to other carriers than the code holder.

Caller ID showing 312-221 is not proof the call came from Chicago Zone 01: spoofing and porting both detach numbers from their origin. To verify a caller, hang up and dial the organization's published number, and report suspicious calls to the FTC.

Neighboring prefixes

FAQ

Where is 312-221 located?

The prefix is assigned to the Chicago Zone 01 rate center in IL, area code 312. It is part of the Chicago-Naperville-Elgin, IL-IN Metropolitan Statistical Area.

Which company owns 312-221 numbers?

T-MOBILE USA, INC. holds the code, but the prefix is pooled — individual thousands-blocks can be assigned to other carriers, and numbers may be ported to any provider.

Is a call from 312-221 safe to answer?

The prefix itself says nothing about the caller. Legitimate Chicago Zone 01 businesses and spoofed robocalls can both display it. Never share personal data with an unverified inbound caller.