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Local Number Portability (LNP)

Local Number Portability (LNP)

Local Number Portability is the system that allows telephone subscribers to keep their phone number when they switch carriers. Before LNP, your phone number was tied to your carrier — if you wanted to switch from Verizon to AT&T, you had to get a new number. LNP broke that link, and in doing so, fundamentally changed how calls are routed in North America.

Why LNP Exists

The Telecommunications Act of 1996 mandated that local exchange carriers implement number portability. The reasoning was straightforward: if consumers could not take their numbers with them, the cost of switching carriers was artificially high. Number portability removed that barrier and enabled real competition in local telephone service.

The FCC set implementation deadlines starting in 1997 for the largest markets. By 2003, wireless carriers were also required to support porting (wireless LNP, or WLNP). Today, virtually every carrier in North America — wireline, wireless, VoIP, and cable — participates in the LNP system.

The Problem LNP Solves

Before portability, NPA/NXX data told you everything you needed to know to route a call. If NXX 555 in area code 312 was assigned to AT&T Illinois, then every subscriber with a 312-555-XXXX number was served by AT&T Illinois, and you could route to AT&T’s switch to reach them.

After portability, that assumption breaks. A subscriber at 312-555-1234 might have ported their number to T-Mobile, Comcast, or a VoIP provider. The NPA/NXX assignment data still says AT&T, but the number actually terminates on a completely different carrier’s network. Routing to AT&T would fail — or worse, AT&T would have to forward the call to the correct carrier, adding cost and delay.

LNP needed a real-time lookup system to tell originating carriers where ported numbers actually live.

The NPAC

The Number Portability Administration Center (NPAC) is the central database that tracks every ported telephone number in North America. It is operated by iconectiv (formerly Telcordia/Bellcore).

When a number is ported from one carrier to another, the new carrier (the “recipient”) submits a port request. Once completed, the NPAC record is updated with the LRN (Location Routing Number) of the switch where the number now terminates.

The NPAC does not handle real-time queries directly. Instead, it distributes its data to LSMS (Local SMS — Service Management System) nodes operated by each carrier. Carriers maintain local copies of the portability database and query their own LSMS for routing lookups.

Location Routing Number (LRN)

The LRN is the key technical mechanism that makes portability work. An LRN is a 10-digit number (formatted like a phone number: NPA-NXX-XXXX) that identifies a specific switch or point of interconnection. When a number is ported, the NPAC record maps the subscriber’s phone number to the LRN of the recipient carrier’s switch.

For example:

Subscriber Number Original Carrier Ported To LRN
(312) 555-1234 AT&T Illinois T-Mobile (312) 601-0000

The LRN tells the originating carrier: “Do not route to AT&T based on the NPA/NXX. Instead, route to the switch identified by LRN 312-601-0000, which belongs to T-Mobile.”

LRNs are assigned from real NPA/NXX blocks, but the subscriber digits (XXXX) in an LRN are often all zeros — the LRN identifies the switch, not an individual line.

The LRN Dip

On every call to a wireline number, the originating carrier performs an LRN dip — a database query against its local copy of the NPAC data. This happens after digit analysis but before trunk selection, as described in How a Phone Call Gets Routed.

The query is simple: “Has this 10-digit number been ported? If so, what is the LRN?”

  • No record found: The number has not been ported. Route based on the original NPA/NXX assignment.
  • LRN returned: The number has been ported. Route to the switch identified by the LRN.

The LRN dip typically takes 10-50 milliseconds. It adds negligible delay to call setup but is essential for correct routing. Every carrier in the call path may perform its own LRN dip, though in practice the originating carrier’s dip is the critical one.

On the PSTN, the originating switch queries the LSMS via SS7 TCAP (Transaction Capabilities Application Part) messages. On VoIP networks, the query is typically made via a proprietary interface to the carrier’s number portability database, with the result used to modify the SIP INVITE before forwarding.

The Porting Process

When a subscriber requests to port their number to a new carrier:

1. Customer Initiates

The subscriber contacts their new (gaining) carrier and requests a port. They provide their current phone number, account information, and authorization.

2. Port Request Submitted

The gaining carrier submits a Local Service Request (LSR) to the losing (donor) carrier. The LSR includes the subscriber’s number, the gaining carrier’s information, and the requested port date.

3. Validation

The losing carrier validates the request — verifying that the subscriber information matches their records and that there are no contractual issues (e.g., early termination). If there are problems, the losing carrier sends a rejection (a “jeopardy” notification).

4. FOC (Firm Order Confirmation)

Once validated, the losing carrier issues an FOC, confirming the port and setting a date and time for activation.

5. Activation

On the scheduled date, the gaining carrier activates the number on their network and updates the NPAC with the new LRN. The NPAC distributes the updated record to all carriers’ LSMS databases. From this moment, all new calls to the number will be routed to the gaining carrier.

6. Disconnect

The losing carrier removes the number from their switch. The old routing is superseded by the NPAC record.

Timelines

The FCC mandates porting timelines:

  • Simple wireline ports: 1 business day
  • Wireless ports: 1 business day (often completed within hours)
  • Complex ports (multi-line business accounts): Up to several business days, negotiated between carriers

In practice, wireless and simple VoIP ports often complete in 2-4 hours. Wireline ports involving physical infrastructure changes may take the full business day.

Types of Porting

Wireline-to-Wireline

The original LNP scenario. A subscriber moves their landline number from one carrier to another. Both carriers must serve the same rate center — you generally cannot port a number to a carrier that does not have facilities in the same geographic area.

Wireless-to-Wireless

Wireless LNP (WLNP), mandated in 2003. A subscriber switches mobile carriers and keeps their number. No geographic restriction — wireless numbers can port between any carriers in the same rate center or broader geographic area.

Wireline-to-Wireless and Vice Versa

Intermodal porting allows numbers to move between wireline and wireless carriers. A business can port its landline number to a mobile phone, or a consumer can port a mobile number to a VoIP provider. This flexibility has been critical for VoIP adoption — businesses can move to cloud phone systems without losing established phone numbers.

Impact on Routing Data

LNP adds a layer of complexity to NPA/NXX data. The carrier shown in NXX assignment data is the original block holder — the carrier that was assigned the NXX block by NANPA. But for any individual number within that block, the actual serving carrier may be different due to porting.

This means:

  • NPA/NXX data shows assignment, not current service: When you see that NXX 555 in area code 312 is assigned to AT&T, that means AT&T was assigned the block. Individual numbers within 312-555-XXXX may have been ported to dozens of different carriers.
  • LRN data is needed for accurate per-number routing: Carriers must perform LRN dips for routing. NPA/NXX data alone is insufficient.
  • Rate center matters for portability: Numbers can generally only be ported within the same rate center or numbering plan area. The rate center data in NPA/NXX records defines the geographic scope of portability.

Our reference data shows the NPA/NXX-level assignments — the original block holder for each exchange. This is the foundation layer. The LRN/porting layer sits on top of it, modifying routing for individual numbers as they port between carriers.

LNP by the Numbers

The scale of number portability in North America is enormous:

  • Over 600 million porting transactions have been processed since LNP began
  • Approximately 40-50 million numbers are ported annually
  • The NPAC database contains records for hundreds of millions of ported numbers
  • Every carrier in the NANP maintains a local copy of this data for real-time routing lookups

Further Reading

The North American Numbering Plan (NANP)Area Code Overlays and Splits