Additional Mobile Subscriber Integrated Services Digital Network Number

Description

The Additional MSISDN (A-MSISDN) is a specialized identifier defined within 3GPP security specifications, primarily in TS 33.108, which details lawful interception (LI) architecture and requirements. It functions as a secondary, administratively assigned Mobile Subscriber Integrated Services Digital Network Number that is associated with a user's subscription alongside their primary MSISDN. From a network architecture perspective, the A-MSISDN is provisioned within the Home Subscriber Server (HSS) or a similar subscriber data repository. It is linked to the user's International Mobile Subscriber Identity (IMSI) and primary MSISDN, creating a mapping that is crucial for the Lawful Interception function. The core network elements, such as the Mobile Switching Center (MSC), Serving GPRS Support Node (SGSN), and Mobility Management Entity (MME), are configured to recognize and handle this identifier when performing interception-related activities.

Operationally, when a lawful interception warrant is activated for a target, the network can be instructed to use the A-MSISDN as the reference identifier for all interception-related communications and data reporting sent to the Law Enforcement Monitoring Facility (LEMF). The Intercepting Control Element (ICE) within the network, such as a Gateway GPRS Support Node (GGSN) or Packet Data Network Gateway (PGW), will utilize the A-MSISDN when generating the Intercept Related Information (IRI) and Content of Communication (CC) reports. This mechanism ensures that the target's actual, public-facing MSISDN is not disclosed in the interception data streams that are delivered to external agencies. The handling of the A-MSISDN is transparent to the end-user and does not affect normal service delivery; the user continues to use their primary MSISDN for all regular calls and data sessions.

The technical implementation involves secure interfaces within the 3GPP LI architecture, namely the Handover Interfaces HI1, HI2, and HI3. The A-MSISDN is primarily used on the HI1 interface, which carries administrative and warrant information, and the HI2 interface, which delivers the IRI. Its role is critical in separating the identity used for network service provision from the identity used for secure reporting to law enforcement. This separation is a fundamental security and privacy principle within lawful interception systems, preventing the exposure of the subscriber's operational identity during the interception process and maintaining the confidentiality of the investigation.

Purpose & Motivation

The A-MSISDN was introduced to address specific operational security and privacy requirements within lawful interception frameworks. Prior to its standardization, interception activities typically used the subscriber's primary MSISDN as the key identifier in reports sent to law enforcement. This practice posed a risk, as the exposure of the actual phone number in interception data streams could compromise an investigation if the data were intercepted or mishandled. Furthermore, it did not align with principles of data minimization and subscriber privacy, even within a lawful context.

The creation of the A-MSISDN provides a method to anonymize or pseudonymize the target's identity within the interception supply chain. Its purpose is to solve the problem of directly exposing the target's public identifier (the MSISDN) to the Law Enforcement Agency (LEA) and its systems. By using an administratively assigned, alternative number for all interception-related reporting, the network protects the integrity of the investigation and safeguards the subscriber's primary identifier. This is particularly important in scenarios involving sensitive investigations or the protection of undercover operations.

Historically, as lawful interception requirements evolved with 3G, 4G, and 5G systems, the need for more sophisticated identity management became apparent. The A-MSISDN concept formalizes a best practice for identity separation, ensuring that 3GPP networks comply with stringent regional regulations on privacy and data protection even while fulfilling legal obligations for interception. It addresses the limitation of the previous approach by decoupling the service identity from the interception identity, thereby enhancing the overall security posture of the lawful interception system.