Ported Number (Mobile Station International Subscriber Directory Number)

Description

A ported number is defined in 3GPP TS 21.905 and TS 22.066 as a Mobile Station International Subscriber Directory Number (MSISDN) that has undergone the porting process. In mobile telecommunications, the MSISDN is the primary identifier by which a subscriber is reached — it is the telephone number dialed by other users. Number portability allows a subscriber to keep this number when switching from one network operator to another, which is a regulatory requirement in many jurisdictions worldwide.

The concept of a ported number is central to the implementation of Mobile Number Portability (MNP). When a subscriber initiates a porting request, the donor operator (the original network) and the recipient operator (the new network) coordinate through a series of administrative and technical procedures. Once the porting process completes, the MSISDN is no longer associated with the donor network's subscriber database but is instead routed to the recipient network. The number itself remains the same from the calling party's perspective, but the underlying routing and subscription data change.

From a technical standpoint, the ported number requires that call routing mechanisms be adapted. In circuit-switched networks, this involves the use of IN (Intelligent Network) services and databases such as the Number Portability Database (NPDB) or the Central Reference Database. When a call is placed to a ported number, the originating network queries the NPDB to determine the current serving network. The call is then routed accordingly. In all-IP networks such as IMS and VoLTE, similar mechanisms exist using ENUM (E.164 Number to URI Mapping) and SIP routing, where the ported number is resolved to the correct serving entity.

The ported number is also significant in charging and billing systems. Since the number has moved between operators, interconnect agreements and settlement procedures must account for the changed routing. Regulators typically mandate maximum porting times and define the rights of subscribers regarding ported numbers. The 3GPP specifications 21.905 and 22.066 provide the vocabulary and service requirements for number portability, ensuring consistent terminology across implementations.

In summary, a ported number is a fundamental identifier in mobile networks that embodies the regulatory and technical framework of number portability. It ensures subscriber mobility across operators without the inconvenience of changing phone numbers, and it requires coordinated database, routing, and signaling mechanisms to function correctly.

Purpose & Motivation

The concept of a ported number exists to support Mobile Number Portability (MNP), a regulatory and consumer-protection measure that allows subscribers to retain their telephone number when changing network operators. Before number portability was introduced, subscribers who switched operators had to change their phone number, which caused significant inconvenience: they had to inform all contacts, update business cards, and potentially lose customers or social connections. This created a barrier to competition, as subscribers were effectively locked into their existing operator to avoid the hassle of changing numbers.

Regulators worldwide recognized that number portability promotes competition by removing this barrier. When subscribers can freely move between operators while keeping their number, operators must compete on price, quality of service, and innovation rather than relying on subscriber inertia. The 3GPP specifications 21.905 and 22.066 were developed to standardize the terminology and service requirements for number portability, ensuring that implementations across different networks and countries are interoperable.

The ported number concept also addresses technical challenges in call routing. Without a standardized definition and associated procedures, networks would not know how to route calls to numbers that have moved. The ported number, as a defined term, enables consistent handling across circuit-switched and packet-switched domains, including IMS and VoLTE. It also facilitates interconnection and settlement between operators, as the ported number indicates which network is currently serving the subscriber.