Mobile Station International Subscriber Directory Number

Description

The Mobile Station International Subscriber Directory Number (MSISDN) is the primary, internationally standardized identifier for a mobile subscriber within the Public Switched Telephone Network (PSTN) and the Integrated Services Digital Network (ISDN). It follows the ITU-T E.164 numbering plan, comprising a Country Code (CC), a National Destination Code (NDC) which often identifies the mobile network operator, and a Subscriber Number (SN). This number is not stored on the SIM card itself but is a key data element within the subscriber's profile in the Home Location Register (HLR) in GSM/UMTS networks or the Home Subscriber Server (HSS) in LTE/5G networks. The MSISDN is decoupled from the device's hardware identity (IMEI) and the SIM's identity (IMSI), allowing a subscriber to change devices while retaining their number and enabling number portability between operators.

In network operations, the MSISDN is the principal address used for routing mobile-terminated voice calls and SMS messages. When a call is placed to an MSISDN, the originating network queries the appropriate number portability database and then routes the call to the subscriber's home network. The home network's Gateway MSC (GMSC) uses the MSISDN to interrogate the HLR/HSS to obtain the current routing information, specifically the Mobile Station Roaming Number (MSRN) or the IP address of the serving MSC or SMSC. This process enables the call or message to be delivered to the subscriber's current location, whether in the home network or roaming.

The MSISDN's role extends beyond basic circuit-switched services. In the IP Multimedia Subsystem (IMS), the MSISDN can be used as a Public User Identity (IMPU) for SIP-based services like VoLTE and ViLTE, allowing interoperability with legacy PSTN/PLMN users. It is also a critical key for subscriber management, billing, and lawful interception. The separation of the MSISDN from the IMSI provides operational flexibility; for example, a single IMSI (and thus a single subscription) can be associated with multiple MSISDNs for services like voice and fax, or for corporate lines. Its standardized, globally routable format is foundational for global mobile telecommunications.

Purpose & Motivation

The MSISDN was created to provide a standardized, user-friendly, and globally unique addressing scheme for mobile subscribers, analogous to telephone numbers in the fixed network. Prior to cellular standardization, various incompatible radio systems used different identification methods, hindering interoperability and international roaming. The adoption of the E.164-based MSISDN within GSM and subsequent 3GPP technologies solved this by providing a consistent number that users could share and that networks could use for routing calls worldwide.

Its design separates the subscriber's public identity (MSISDN) from their private network identity (IMSI) and equipment identity (IMEI). This separation is crucial for security, privacy, and operational flexibility. It allows for subscriber mobility and number portability without changing the physical SIM card or device. The MSISDN enables the mobile network to interface seamlessly with the global PSTN/ISDN, making the mobile phone a true extension of the worldwide telephone system. It solved the fundamental problem of how to find and route a call to a mobile subscriber whose location is not fixed, by using it as a stable key to query dynamic routing information from the HLR.