Commonly confused terms

Pairs people mix up all the time — side by side, with the tell that separates them.

Overlay vs Split Original Assignee vs Current Service Provider NXX (prefix) vs Thousands-Block Caller ID vs CNAM Area Code vs Rate Center

Central Office Code (NXX)

The three-digit prefix after the area code (the "NXX" in NPA-NXX-XXXX), historically identifying a telephone switch.

An NXX no longer reliably identifies one physical switch or one carrier. Number pooling splits NXX codes into thousands-blocks held by different providers, and portability means the assignee of record may not serve the number today.

Full definition →

Thousands-Block

A pooled block of 1,000 consecutive numbers within an NXX (e.g. 626-470-0XXX) — under pooling, the unit numbers are allocated in.

A thousands-block being "assigned" describes an allocation to a carrier, not that all 1,000 numbers are in use by subscribers. Prefix- and block-level assignment is not the same as number-level utilization.

Full definition →

A number is country code + area code (NPA) + prefix (NXX) + a four-digit line number. The first digit of the four-digit line number — the seventh digit of the 10-digit national number, not counting the country code +1 — marks which pooled thousands-block of 1,000 numbers it belongs to.

How to tell them apart. The tell: an NXX is 10,000 numbers (one prefix); a thousands-block is 1,000 of them. Under pooling, one NXX can be shared by several carriers, one block each.