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Routing
How a Phone Call Gets Routed

How a Phone Call Gets Routed

When someone dials a phone number, a complex chain of lookups, signaling exchanges, and trunk selections happens in milliseconds. This article walks through the complete lifecycle of a phone call in North America — from the moment digits are dialed to the moment the called party’s phone rings — covering both traditional PSTN and modern VoIP paths.

The Example Call

To make this concrete, we will follow a call from a subscriber in Chicago (312 area code) calling a number in northern New Jersey (area code 201). The caller dials (201) 555-7890.

Step 1: Digit Collection and Analysis

The caller’s device (landline phone, mobile handset, or VoIP softphone) sends the dialed digits to the originating switch — the first piece of carrier equipment that handles the call.

On the PSTN, this is a Class 5 switch (the local central office switch). On a VoIP network, this is typically a softswitch or SBC (Session Border Controller) operated by the caller’s carrier.

The switch performs digit analysis: it examines the dialed string to determine what kind of call this is and where to route it.

  • 1 + 10 digits: Long-distance call (the leading 1 is stripped; routing uses the remaining NPA + NXX + XXXX)
  • 10 digits (NPA + NXX + XXXX): Required in all overlay areas; the switch examines the NPA to determine if it is local or long distance
  • 7 digits (NXX + XXXX): Local call within the caller’s home NPA (only in areas without overlays — increasingly rare)
  • N11 codes: Service codes like 911 or 411, routed to special service platforms
  • 0 or 00: Operator assistance

In our example, the Chicago switch sees NPA 201. That is a New Jersey area code — this is an interstate long-distance call.

Step 2: NPA/NXX Lookup

The originating switch consults its routing tables to determine how to reach NPA 201, NXX 555. Every carrier maintains a database mapping NPA/NXX combinations to route entries. This database tells the switch:

  • Which trunk group or SIP route to use for this destination
  • The rate center associated with the NXX (determines billing jurisdiction)
  • The LATA of the destination (LATA 224 for northern New Jersey)
  • Whether the call is local, intraLATA toll, or interLATA long distance

Since our caller is in Chicago (LATA 358) and the destination is in New Jersey (LATA 224), this is an interLATA call, which means it must traverse a long-distance carrier (IXC) or an IP-based transit provider. This LATA distinction dates back to the Bell System breakup and still governs how calls are classified and billed.

Step 3: Number Portability Lookup (LRN Dip)

Before routing the call, the originating carrier must determine whether the called number has been ported — that is, whether the subscriber has moved their number to a different carrier than the one originally assigned the NXX block.

The carrier queries the NPAC (Number Portability Administration Center) database, maintained by iconectiv. This lookup is called an LRN dip (Location Routing Number dip).

The NPAC returns one of two results:

  • No port record: The number is still with the original carrier. The switch routes based on the NPA/NXX assignment data.
  • LRN returned: The number has been ported. The LRN is a 10-digit number (formatted like a phone number) that identifies the switch where the ported number now terminates. The originating carrier routes to this LRN instead of the original NPA/NXX destination.

For example, if (201) 555-7890 was originally assigned to Verizon but the subscriber ported to T-Mobile, the NPAC returns a T-Mobile LRN. The call is routed to T-Mobile’s switch, not Verizon’s.

The LRN dip happens on every call to a wireline number and adds only a few milliseconds. It is one of the most critical steps in modern call routing — without it, ported numbers would be unreachable. See Local Number Portability for a deeper dive on how this system works.

Step 4: Carrier and Trunk Selection

With the routing destination determined (either from NPA/NXX tables or from the LRN), the originating switch selects a path to reach the terminating carrier.

Direct Trunking

If the originating carrier has a direct interconnection with the terminating carrier, the call goes straight across. Large carriers like AT&T, Verizon, and T-Mobile have extensive direct peering arrangements. A direct trunk between two carriers is the most efficient path — fewest hops, lowest latency, best quality.

Tandem / Transit Routing

If there is no direct trunk, the call is sent to a tandem switch (PSTN) or transit carrier (VoIP). The tandem acts as an intermediary, receiving the call from the originating carrier and forwarding it to the terminating carrier. A call may pass through one or two tandems in transit.

In the VoIP world, transit carriers like Bandwidth, Lumen, or Telnyx serve the same function — they have interconnections with many downstream carriers and can reach destinations that the originating carrier cannot reach directly.

Least Cost Routing

Carriers with multiple upstream options use least cost routing (LCR) to choose the cheapest path for each call. The LCR engine consults rate decks (pricing per NPA/NXX from each upstream carrier) and selects the route with the lowest per-minute cost, subject to quality constraints. LCR decisions happen in real time and can change call-by-call.

Step 5: Signaling — Setting Up the Call

With a route selected, the originating switch sends a call setup message to the next hop. The signaling protocol depends on the network type.

SS7 Signaling (PSTN)

On the traditional PSTN, call setup uses SS7 (Signaling System 7). The originating switch sends an ISUP IAM (Initial Address Message) over the SS7 signaling network. The IAM contains:

  • Called party number (201-555-7890)
  • Calling party number (312-XXX-XXXX) — caller ID
  • Nature of connection indicators
  • Forward call indicators (e.g., ISDN user part preference)

The SS7 signaling network is a separate packet network that runs alongside (but independent of) the voice trunks. Signaling messages travel through STPs (Signal Transfer Points) to reach the destination switch.

SIP Signaling (VoIP)

On IP networks, call setup uses SIP (Session Initiation Protocol). The originating SBC sends a SIP INVITE to the next-hop carrier’s SBC. The INVITE contains:

INVITE sip:+[email protected] SIP/2.0
Via: SIP/2.0/TLS sbc-chicago.carrier-a.com:5061
From: <sip:+[email protected]>;tag=abc123
To: <sip:+[email protected]>
Call-ID: [email protected]
CSeq: 1 INVITE
Contact: <sip:+[email protected]:5061>
Content-Type: application/sdp

v=0
o=- 123456 654321 IN IP4 198.51.100.10
s=phone call
c=IN IP4 198.51.100.10
t=0 0
m=audio 20000 RTP/AVP 0 8 101
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:101 telephone-event/8000

The SDP (Session Description Protocol) body describes the media capabilities — codecs offered, IP address and port for RTP audio, and DTMF handling.

Interworking

Many calls today traverse both networks. A call may originate on a VoIP platform, transit over SIP trunks, hit a media gateway that converts to TDM, cross an SS7-signaled trunk to a legacy switch, and terminate on a copper line. The media gateways and SBCs at each boundary handle the protocol translation transparently.

Step 6: Transit and Intermediate Switching

If the call passes through a tandem switch or transit carrier, the signaling is relayed hop by hop. Each intermediate node:

  1. Receives the call setup message (IAM or INVITE)
  2. Performs its own routing lookup to determine the next hop
  3. Forwards the signaling to the next carrier
  4. Reserves a voice channel or RTP media path

In a typical interLATA call like our Chicago-to-New-Jersey example, the path might be:

Caller's phone
  → Originating carrier switch (Chicago)
    → Long-distance transit carrier
      → Terminating carrier switch (New Jersey)
        → Called party's phone

Each arrow represents both a signaling hop and a media (voice) path segment.

Step 7: Terminating Switch and Ringing

The signaling finally reaches the terminating switch — the carrier equipment serving the called party at (201) 555-7890.

The terminating switch:

  1. Validates that the called number exists and is in service
  2. Determines the subscriber’s line or device (physical copper pair, wireless handset, or registered SIP endpoint)
  3. Sends ringing current to a landline, a push notification to a mobile, or a SIP INVITE to a registered VoIP device
  4. Sends back a ringing indication to the originating side:
    • SS7: ACM (Address Complete Message) indicating the called party is being alerted
    • SIP: 180 Ringing provisional response

The caller hears ringback tone. This tone is generated locally by the originating switch (or the caller’s device) — it is not the actual ringing signal at the far end.

Step 8: Answer and Media Path

When the called party picks up:

  1. The terminating switch sends an answer signal:
    • SS7: ANM (Answer Message)
    • SIP: 200 OK (followed by ACK from the originating side)
  2. The voice path is now fully established — a bidirectional audio channel exists between the two parties
  3. Billing starts — answer supervision is the trigger for call duration metering on both the originating and terminating sides

On TDM networks, the voice path is a 64 kbps DS0 channel through each trunk in the path. On IP networks, audio flows as RTP (Real-time Transport Protocol) packets, typically using the G.711 codec at 64 kbps or a compressed codec like G.729 at 8 kbps.

Step 9: Call Teardown

When either party hangs up:

  1. The side that hangs up sends a release signal:
    • SS7: REL (Release Message), answered by RLC (Release Complete)
    • SIP: BYE request, answered by 200 OK
  2. Each switch in the path releases the trunk/channel resources
  3. Billing stops — call duration is calculated from answer to release
  4. CDRs (Call Detail Records) are written by each carrier in the path for billing and record-keeping

The entire teardown completes in milliseconds.

The Role of NPA/NXX Data

Every step in this process depends on accurate, current NPA/NXX data:

  • Digit analysis uses NPA data to classify the call (local, toll, interstate)
  • Routing tables use NPA/NXX assignments to select trunk groups and carriers
  • LRN dips determine if the NPA/NXX-level routing needs to be overridden for a ported number
  • Rate decks price each call based on the NPA/NXX of the destination
  • Billing systems use NPA/NXX-to-rate-center mappings for jurisdictional classification

This is why carriers update their routing data continuously, and why stale NPA/NXX data causes real operational problems — misrouted calls, incorrect billing, and unreachable numbers.

Modern Simplifications

While the full routing chain described above still exists, several trends are simplifying the picture:

  • All-IP networks eliminate the TDM/SS7 segments for many calls, reducing the call to a chain of SIP hops
  • Carrier consolidation means fewer transit hops — the large carriers (AT&T, Verizon, T-Mobile, Lumen) can often complete calls within their own networks
  • STIR/SHAKEN adds caller ID authentication to the signaling chain, helping combat robocall spoofing
  • Cloud PBX and UCaaS platforms handle much of the complexity internally, presenting a simpler interface to businesses and end users

But underneath all of these layers, the fundamental routing logic — NPA/NXX lookup, LRN dip, trunk selection, signaling, answer supervision, teardown — remains the same architecture that has powered North American telephony for decades.

Further Reading

Least Cost Routing (LCR) Explained