Thousands-Block Pooling & Rate Center Architecture
How NANPA, Somos, and LERG rate centers determine who controls a block of 1,000 phone numbers.
The 10-Digit Hierarchy
North American phone numbers follow the format NPA-NXX-XXXX. The NPA is the Area Code (3 digits). The NXX is the Central Office switch code (3 digits). The remaining 4 digits represent the individual line.
Historically, a carrier wanting to serve even one subscriber in a town was assigned an entire NXX — 10,000 numbers at once. This created massive artificial number exhaustion in the 1990s.
Thousands-Block Pooling
Under modern pooling rules, NXX blocks are divided into ten 1,000-number blocks (NPA-NXX-0 through NPA-NXX-9). Block holders are designated in the Local Exchange Routing Guide (LERG).
When monitoring an old number for re-acquisition, identifying the OCN (Operating Company Number) of the specific thousands-block is the single most critical step in knowing which carrier will eventually re-issue it.
Looking to recover or track a specific phone number?
Use our free reverse lookup engine to audit its thousands-block allocation, or subscribe to automated monitoring for instant pool return alerts.