LRN lookup API

LRN lookup API: routing number and porting status for US and Canadian numbers

An LRN (Local Routing Number) is the 10-digit number that identifies the switch serving a phone number. When a number is ported to another carrier, calls and texts are routed on its LRN, not on its own digits. The CheckThatPhone LRN lookup API returns that routing number for any US or Canadian number as dipLrn, together with dipPorted, dipOcn, dipCarrier, dipCarrierType and dipCarrierSubType, from a live carrier dip at lookup time.

The FCC's number portability orders call it the location routing number: each central office switch is assigned a unique 10-digit number that "serves as a network address" for routing (FCC 97-289, para. 8). Number portability itself is the ability to keep your number when you switch carriers at the same location (47 CFR 52.21). One POST /v1/lookup call costs one credit, and the Free plan includes 500 lookups a month.

Want the carrier name for display, filtering or line type? The carrier lookup API page covers which carrier serves a number. This page is about the routing side of the same lookup: the LRN, the ported flag and the OCN.

Fields an LRN lookup returns

A lookup returns these routing fields. Values are strings, including booleans.

Ported vs original carrier

Phone numbers are handed to carriers in blocks, by area code and prefix (the NPA-NXX). A prefix table can tell you which carrier a block was assigned to, and an area code tells you where the number was issued: see the area code directory. Neither tells you who serves the number today. When a subscriber ports, the number keeps its area code and prefix and moves to a new carrier, so any answer built on the digits alone still points at the original block holder.

The live carrier dip reads current routing data instead. For a ported number, dipLrn holds the routing number of the new carrier's switch, dipPorted is "true", and dipCarrier and dipOcn name the carrier the number moved to. The numbering-plan view is in nanpCarrier, which the docs flag as not authoritative after a port: prefer dipCarrier. The carrier lookup API page goes deeper on the carrier fields.

Example request and response

Send the number in any common format.

curl --request POST \
  --url 'https://api.checkthatphone.com/v1/lookup' \
  --header 'Authorization: Bearer YOUR_API_KEY' \
  --header 'Content-Type: application/json' \
  --data '{"phone": "469-555-0163"}'

An illustrative response for a ported mobile number (a fictional 555 number; field names and formats match the live API). dipLrn differs from subscriber, so dipPorted is "true":

{
  "success": true,
  "credits_used": 1,
  "data": {
    "subscriber": "4695550163",
    "optDate": "2026-10-08T16:21:09.514Z",
    "action": "send",
    "deliverable": "true",
    "nanpType": "mobile",
    "blackList": "false",
    "dip": "success",
    "dipLrn": "4695550198",
    "dipPorted": "true",
    "dipOcn": "6529",
    "dipCarrierSubType": "PCS",
    "dipCarrier": "T-Mobile",
    "dipCarrierType": "mobile",
    "geoState": "TX",
    "geoCity": "dallas",
    "geoCountry": "US",
    "timezone": "America/Chicago",
    "tzOffset": 6,
    "geoSource": "area-code",
    "error": "false",
    "smsEligible": "true"
  }
}

credits_used: 1 is the whole cost of this call. The same fields come back from the official Node.js and Python SDKs and in every row of a bulk CSV upload.

What teams use LRN and porting data for

• SMS and voice routing. If you pick a route or a carrier connection per destination, you need the network that serves the number now. dipLrn identifies the serving switch and dipOcn the carrier, so a ported number goes to its new network instead of the one its prefix suggests.
• Least-cost routing and rating. Voice rate decks are often keyed by NPA-NXX. Rating a ported number on its own digits prices the call as if it ended on the original carrier's network; rating on its LRN prices it by the switch that will actually take it.
• Fraud signals from porting. Porting is common and usually innocent (people change carriers), so treat dipPorted: "true" as one input, not a verdict. It carries more weight stacked with other signals, such as a VoIP carrier (dipCarrierSubType: "IPES") on a brand-new account. Storing dipCarrier and dipOcn at signup and re-checking on a sensitive action shows whether the number has changed carriers since you last looked. KYC phone verification walks through it.
• Keeping carrier records current. Re-checking a list on a schedule catches numbers that moved carriers since they were collected. Upload a CSV to validate a whole list.

Limits worth knowing

• No porting date or history. You get the current routing number, carrier and ported flag. We don't return when a number was ported or which carriers it passed through.
• No SIM-swap detection. A SIM swap normally keeps the number on the same carrier, so the LRN and carrier don't change.
• Porting is not reassignment. A ported number usually still belongs to the same person; a reassigned one belongs to someone new. A lookup can't tell you who holds a number. For recent carrier deactivations, see the disconnected number check.
• US and Canada only. NANP numbers. For other countries, use a global provider.

Pricing

LRN and porting data are part of every lookup: one credit, no separate fee. The Free plan has 500 lookups a month, hard-capped. Paid plans are $5 for 1,000, $30 for 10,000, $100 for 100,000 and $450 for 500,000 lookups a month, with overage from $0.005 down to $0.0009 per lookup. Failed lookups are free. See pricing.

Related

• Field reference: carrier dip fields in the docs
• Which carrier: Carrier lookup API
• Mobile, landline or VoIP: Line type lookup API
• Recent disconnections: Disconnected number check API
• Background: What is a carrier lookup?
• Background: The North American Numbering Plan
• Fraud scoring: Vetting inbound leads: the phone signals that flag obvious fraud