US number portability is a genuine benefit to consumers and a quiet trap for lazy billing. It lets a person or business keep their number when they change carriers — and in doing so, it severs the once-reliable link between a number’s prefix and the carrier that actually serves it. Any wholesale provider that still rates and routes traffic on the assumption that a number lives where its prefix says it does is making a mistake that shows up, eventually, as a billing dispute or a misroute. Getting this right requires two different lookups doing two different jobs, applied to both numbers on every call.
Two lookups, two jobs
The LERG and the LRN dip are often mentioned in the same breath, but they answer different questions and neither substitutes for the other.
- The LERG gives you the static picture: for a fixed NPA-NXX block, which LATA, OCN, rate center, and state it belongs to. It is how you determine the jurisdiction of a call — interstate, intrastate, or local — which drives how it is rated.
- An LRN dip gives you the live picture: for a specific number that may have ported, which switch and carrier it actually routes to today. It is how you find out where a number really lives, independent of its original prefix.
The relationship between them is the key insight. The LERG tells you the jurisdiction of a number block; the LRN dip tells you which block a specific ported number now behaves as though it belongs to. You dip the LRN to find the number’s true home, then run that result back through the LERG to determine jurisdiction. Skip the dip, and you are applying LERG data to a prefix that may no longer mean what it says.
Why both numbers, not just one
It is not enough to dip the called number. Correct rating dips both the calling number (ANI) and the called number (DNIS), because either can be a ported number, and both can affect how a call is rated and treated. The calling number’s true jurisdiction matters for how the call is classified; the called number’s true routing matters for where it goes and what it costs. Dipping only one leaves the other on a potentially stale prefix — which is exactly the gap that produces mis-rated intrastate traffic, the category where the money is.
What goes wrong without it
The failure modes are not dramatic; they are quiet and cumulative. A number that ported from a rural intrastate exchange to a wireless carrier may bill as interstate if you trust its prefix, or as intrastate if you dip it — and the difference, multiplied across millions of minutes, is real money moving in the wrong direction. Routing decisions made on stale data send calls down the wrong path. And when a customer disputes an invoice, a provider that cannot show it dipped the numbers has no defensible basis for its rating. Correct dipping is more work per call; it is also the difference between billing that reconciles and billing that starts arguments.
Immutability matters too
There is a subtle discipline here worth naming. LRN dip data changes over time — numbers port, records refresh, TTLs expire. But a call detail record must be immutable: the rating on a call should reflect what was true at the moment the call was made, not what a lookup returns weeks later. That is why serious platforms snapshot the LRN dip result onto the CDR at call time rather than referencing a live, changing table. The dip is a live question; the record of the answer is permanent.
Hadlo dips both the calling and the called number, independently, on every call, then runs the result back through a real LERG table for jurisdiction — and snapshots the result onto an immutable CDR. It is more rigorous than many commercial platforms bother with, and it is exactly why our billing reconciles. If accurate rating and routing matter to your traffic, talk to us.