How are phone numbers routed between carriers?

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Phone numbers are routed between carriers through a complex and highly interconnected global system, primarily involving the Public Switched Telephone Network (PSTN) and, increasingly, Voice over Internet Protocol (VoIP) networks. The process ensures that a call initiated on one network can reach a subscriber on another, regardless of their geographical location or specific service provider.

Here's how inter-carrier routing works:

Interconnect Agreements:

The fundamental basis for routing calls between different carriers is interconnect agreements. These are commercial and technical agreements between telecommunication operators that define the terms, conditions, and technical specifications for exchanging traffic (voice, SMS, data) between their respective networks.
These agreements specify the Points of Interconnection (PoIs) – the physical or virtual interfaces where the networks connect and exchange signaling and media traffic.
Interconnect agreements also involve settlement rates, which are the fees one carrier pays another for terminating calls on its network.
Numbering Plan and Routing Tables:

Every country has a National Numbering Plan (NNP), managed usa student phone number list by its National Regulatory Authority (e.g., BTRC in Bangladesh, FCC in the US). This plan dictates the structure of phone numbers, including country codes, mobile network codes (MNCs), area codes (for fixed lines), and subscriber numbers.
Carriers maintain routing tables within their switching systems (e.g., Mobile Switching Centers for mobile networks, Class 4/5 switches for fixed lines). These tables contain rules and paths to route calls based on the dialed number.
When a call is initiated, the originating carrier's switch analyzes the dialed phone number to determine the destination network based on its prefix (e.g., the first few digits indicate the carrier or geographic area).
Signaling System No. 7 (SS7) for Call Setup:

The actual "handshake" and information exchange between carriers for call setup and routing traditionally occurs over the Signaling System No. 7 (SS7) network. SS7 is an out-of-band signaling protocol, meaning the control information travels on a separate channel from the actual voice traffic.
When a user dials a number on Carrier A, Carrier A's switch sends SS7 messages to identify the destination network.
If the destination is on Carrier B's network, Carrier A sends a routing query (e.g., a SendRoutingInfo message) to Carrier B's network to obtain the necessary information (like the current VLR address for a mobile subscriber) to route the call.
Number Portability (MNP/LNP) and Central Databases:

The introduction of Mobile Number Portability (MNP) and Local Number Portability (LNP), which allow subscribers to keep their phone numbers when switching carriers, adds a layer of complexity to routing.
Since the original prefix of a ported number still belongs to the "donor" (original) carrier, directly routing based on the prefix would fail.
To overcome this, most countries operate a centralized Number Portability Administration Center (NPAC) or database.
When a call is made to a potentially ported number, the originating carrier's switch queries this NPAC (or a replicated local database derived from it) to determine the current serving carrier for that specific phone number. This process is often called an HLR lookup or MNP lookup.
Once the NPAC provides the identity of the actual serving carrier (the "recipient" carrier), the call is then routed directly to that network.
Voice Path Establishment (Media Flow):

After the signaling (via SS7) has determined the optimal path and successfully set up the call, the actual voice (media) traffic flows over the established circuit or IP path between the originating and terminating carriers' networks.
This can involve traditional Time-Division Multiplexing (TDM) circuits for PSTN-to-PSTN calls or Session Initiation Protocol (SIP) trunks for VoIP-to-VoIP calls or calls transiting between TDM and IP networks.
In essence, phone numbers are routed between carriers through a sophisticated choreography of commercial agreements, centrally managed numbering plans, real-time signaling queries (especially for mobile and ported numbers), and physical/virtual interconnections, all designed to deliver a seamless communication experience.