LRN (Local Routing Number) is a 10-digit number that tells the SS7 and SIP routing infrastructure where to deliver a call or message — not based on where the number was originally assigned, but based on where it lives right now. Without LRN, ported numbers couldn't exist.
What Is an LRN?
Every telephone number in the US PSTN is mapped to an LRN. When a call enters the network, a dip into the NPAC database translates the dialed number into its associated LRN. The LRN then identifies the terminating switch and routes the traffic accordingly.
Every time a subscriber moves their number from one carrier to another, the LRN entry in NPAC updates to point to the new carrier's switch. The phone number stays the same; the LRN changes.
How LRN Works in the SS7/SIP Routing Stack
In SS7 networks, an ISUP Initial Address Message (IAM) carries the LRN after a successful NPAC dip. In SIP-based interconnects, the LRN is passed in the tel: URI or as a rn= parameter in the Request-URI.
The sequence for a ported number:
- Caller dials +1-212-555-0100 (originally AT&T)
- Originating switch performs NPAC dip
- NPAC returns LRN: 2125550001 (T-Mobile switch in NYC)
- Call routes to T-Mobile using the LRN, not the dialed number
- T-Mobile delivers to the subscriber's current handset
What Happens to LRN When a Number Ports
When a subscriber initiates a port, NPAC updates the LRN record — typically within minutes. The old LRN becomes invalid. Any carrier that cached the old LRN will misroute the traffic. For voice, this means calls go to the wrong switch. For A2P messaging, the message hits the wrong aggregator and gets rejected.
LRN Activation Date: A VRI-Exclusive Data Point
Standard LRN lookups return the current LRN and carrier. VRI goes further: alongside every LRN response, VRI returns the LRN Activation Date — the timestamp of when the current LRN became active for that number.
Two primary use cases:
- SIM swap / account takeover detection: A very recent LRN Activation Date (under 7 days) indicates the number recently ported. Fraud teams use this to flag high-risk authentication events. SMS OTP sent immediately after a port likely goes to an attacker, not the legitimate account holder.
- Messaging deliverability: CPaaS providers use LRN Activation Date to determine whether a route refresh is needed.
OCN Codes for Major US Carriers
The LRN maps to a carrier via its Operating Company Number (OCN). Key OCNs for major US carriers:
| Carrier | OCN |
|---|---|
| Verizon Wireless | 6006 |
| AT&T Mobility | 6664 |
| T-Mobile USA | 6529 |
| Lumen (CenturyLink) | 0214 |
| Comcast Phone | 183E |
US Cellular and other regional carriers operate under multiple region-specific OCNs. Use VRI's carrier lookup API to retrieve the current OCN for any specific number.
OCN is carrier-level. It does not identify the messaging provider. For messaging use cases, the messaging provider ID is the operationally relevant field.
Why Stale LRN Causes Misrouted Calls and Failed 10DLC Messages
Port activity in the US runs at roughly 40,000–60,000 numbers per day. For any dataset more than a few hours old, a non-trivial share of numbers will have the wrong LRN.
For voice carriers, stale LRN leads to SIP 404/502 errors and calls landing in wrong carrier IVR. For A2P messaging on 10DLC, stale LRN causes silent drops or 30007 (carrier violation) errors. Real-time LRN lookup before each send eliminates this failure class.
API Example: LRN Lookup with Activation Date
curl -s "https://api.verirouteintel.com/v1/lookup" \
-H "Authorization: Bearer YOUR_API_KEY" \
-G --data-urlencode "phone=+12125550100" \
--data-urlencode "fields=lrn,lrn_activation_date,ocn,carrier,messaging_provider"Response:
{
"phone": "+12125550100",
"lrn": "2125550001",
"lrn_activation_date": "2026-05-12T14:33:00Z",
"ocn": "6529",
"carrier": { "name": "T-Mobile USA", "type": "wireless" },
"messaging_provider": { "id": "TMO", "name": "T-Mobile Messaging" }
}When to Refresh Your LRN Cache
- Numbers not seen in 30+ days: Always re-lookup before sending
- Numbers that generated a delivery error on the last send: Re-lookup immediately
- Numbers flagged as ported in the last 90 days: Re-lookup at least weekly
Look up an LRN now — 10 free lookups, no credit card required. Get started →