Understanding CLLI Codes

What Is a CLLI Code?

A CLLI code (pronounced "silly") stands for Common Language Location Identifier. It is an 11-character alphanumeric code assigned to every physical location that houses telephone network equipment in North America — central offices, tandem switches, data centers, cable landing stations, and wireless interconnection points.

The CLLI standard was developed by Telcordia Technologies (formerly Bellcore, now Ericsson), which maintains the CLLI database and assigns codes to network locations. Every network element registered in the Telcordia LERG (Local Exchange Routing Guide) is identified by a CLLI code. When you look up a phone number and see a location like "NYCMNYBXDS1," that's the CLLI code for the central office serving that number.

CLLI codes are used throughout the telecom industry for:

• Routing tables in the LERG (Local Exchange Routing Guide)
• Carrier interconnection agreements and trouble tickets
• Equipment provisioning and ordering systems
• Physical network inventory management
• Identifying where a phone number's dial tone originates

Anatomy of an 11-Character CLLI Code

Every CLLI code follows a strict 11-character structure. Each group of characters encodes specific information about the location and equipment type:

Common City Codes (Characters 1–4)

Common Equipment Type Codes (Characters 9–11)

Decoding a Real CLLI Code: Step by Step

Let's decode a complete example CLLI code: NYCMNYBXDS1

Now let's decode a switch CLLI: CHCGILXIDM1

Why CLLI Codes Matter for Phone Lookups

When foneinfo.us (or any carrier lookup service) returns information about a phone number, the CLLI code is one of the most specific and reliable pieces of data available. Here's why it matters:

• Location precision: A CLLI code identifies not just the city but the specific building where the switch sits. This is far more precise than just knowing the NPA (area code) or NXX (exchange prefix).
• Switch type identification: The equipment suffix tells you exactly what technology is serving the number — legacy TDM hardware, softswitch, or something else.
• Carrier intelligence: Different CLLI codes at the same location can indicate different carriers sharing a building (colocation). Seeing an unexpected CLLI can reveal porting or wholesale arrangements.
• Routing verification: Carriers use CLLI codes for all provisioning and routing. If a number's CLLI doesn't match what you'd expect based on the area code and carrier, it may indicate porting, number transfer, or a CLEC arrangement.

One important caveat: CLLI codes in routing databases are not always updated immediately when numbers are ported or when switches are replaced. A CLLI showing DM1 (DMS-100) might still appear in the routing database even after the actual switch was replaced by a Metaswitch. This database latency is a known limitation of CLLI-based lookups.

CLLI vs. OCN vs. NPA-NXX

CLLI codes are often confused with other telecom identifiers. Here's how they differ: