Call Related Mobile Number Portability

Description

Call Related Mobile Number Portability (CRMNP) is a standardized service defined by 3GPP that facilitates the routing of voice calls to a mobile subscriber who has changed their service provider while keeping their original Mobile Subscriber Integrated Services Digital Network Number (MSISDN). The core technical challenge it addresses is the decoupling of a subscriber's identifier (the MSISDN) from its original network routing information. In a non-ported scenario, the MSISDN contains an embedded Mobile Network Code (MNC) and Mobile Country Code (MCC) that directly indicates the subscriber's Home Public Land Mobile Network (HPLMN). After porting, this embedded data becomes incorrect for routing purposes.

The architecture for CRMNP relies on a centralized or distributed Number Portability Database (NPDB). When a call is placed to a ported number, the originating network's switching node (like a Mobile Switching Center or Gateway MSC) must perform a number portability query. This query, typically using signaling protocols like Signaling System No. 7 (SS7) or Diameter, is sent to the NPDB. The NPDB, which contains a mapping of all ported numbers to their current serving network's routing number (e.g., a Mobile Station Roaming Number - MSRN, or a specific routing prefix), responds with the correct routing information. The originating network then uses this information to route the call to the recipient's new network.

Key components in the CRMNP process include the Number Portability Database (NPDB), the Signaling Transfer Point (STP) or query routing function, and the network switches (MSC/GMSC). The NPDB is the authoritative source for ported number mappings. The query routing function ensures the query reaches the correct NPDB, which may be organized on a national or regional basis. The performance of this query-and-response mechanism is critical for call setup time, and standards define requirements to minimize post-dial delay. CRMNP is primarily a call control and signaling function, integrated into the core network's call session control layer.

CRMNP's role is to make number portability transparent to the calling party and seamless for the called party. It ensures the public telephony network functions correctly despite the administrative change of a subscriber's network affiliation. The service must handle various call scenarios, including calls from fixed networks, international calls, and calls while the ported subscriber is roaming. The implementation details, such as the query trigger point (All Call Query, Query on Release, etc.) and the specific routing number returned, can vary by national regulation and operator implementation, but the 3GPP standard provides the framework for interoperability.

Purpose & Motivation

CRMNP was created to fulfill regulatory requirements established by governments and telecommunications authorities to promote consumer choice and market competition. Before number portability, subscribers were effectively 'locked in' to their current mobile operator because changing providers meant losing their phone number, a significant inconvenience and barrier to switching. This lack of portability stifled competition, as new market entrants could not easily attract customers from established incumbents.

The primary problem CRMNP solves is the technical routing of calls in a landscape where a subscriber's identifier (MSISDN) no longer inherently points to their actual network. The traditional telephony routing plan was hierarchical and based on the numbering plan, which allocated blocks of numbers to specific operators. CRMNP breaks this direct link, requiring a dynamic lookup mechanism. Its creation was motivated by the need for a standardized, network-agnostic solution that all operators in a market could implement, ensuring that a call to any number would complete correctly regardless of the subscriber's current network provider.

Prior to CRMNP, there was no standardized mechanism for this, which would have led to interoperability chaos or required proprietary, bilateral agreements between operators. By defining CRMNP, 3GPP provided a clear technical specification that enables nationwide or region-wide number portability schemes. This levels the competitive playing field and is considered a fundamental right in most modern telecommunications markets, directly contributing to lower prices, better service quality, and innovation driven by consumer mobility.

Key Features

• Enables retention of MSISDN when changing mobile network operators
• Utilizes a centralized Number Portability Database (NPDB) for routing information
• Defines standardized signaling procedures for querying ported number status
• Supports various implementation models like All Call Query (ACQ)
• Minimizes impact on call setup time through optimized query mechanisms
• Ensures interoperability between different network operators' core networks

Evolution Across Releases

Defining Specifications