Fixed Dialling Number

Description

Fixed Dialling Number (FDN) is a subscriber-centric service feature defined within the 3GPP specifications for GSM, UMTS, and LTE/5GS networks. It functions as a call barring mechanism that restricts the outgoing calls a user can make from a mobile device to only those numbers explicitly listed in an FDN list stored on the Universal Subscriber Identity Module (USIM) or SIM card. The primary enforcement point is the Mobile Equipment (ME) itself, which consults this USIM-resident list before allowing a call setup to proceed. When a user attempts to place a call, the ME's call control software compares the dialed number against the entries in the FDN list. If a match is found (or if the number is on a permitted 'emergency' exception list defined by the network operator or regulator), the call is allowed to proceed through normal signaling to the network. If no match is found, the ME terminates the call attempt locally, and the network is never contacted, preventing any potential charges or unauthorized usage.

The architecture of FDN is tightly integrated with the USIM application toolkit and the device's modem software. The FDN list is stored as a dedicated file (EF_FDN) within the USIM's file system, protected by an access condition, typically a PIN2. Management of the list—adding, deleting, or modifying entries—is performed through the Man-Machine Interface (MMI) of the phone, which sends APDU commands to the USIM to update the EF_FDN file after successful PIN2 verification. This design ensures the security and portability of the restriction list; it moves with the SIM card, applying the same rules regardless of the handset used (provided the handset supports FDN). The service is independent of the core network; it is a client-side feature, though network operators may provision the initial list or enable/disable the service via Over-The-Air (OTA) protocols.

In the broader service architecture, FDN interacts with other call barring services like Barring of All Outgoing Calls (BAOC) and Barring of Outgoing International Calls (BOIC), but it is more granular. While network-based barring services are enforced by the Mobile Switching Center (MSC) or Access and Mobility Management Function (AMF)/Session Management Function (SMF) based on subscriber profile, FDN provides user- or administrator-defined control at the device level. Its role is crucial for specific use cases: it allows parents to limit a child's phone to contacts, enables companies to restrict corporate devices to business numbers, and helps prevent fraud by locking stolen SIMs to a limited set of numbers. Despite being a legacy GSM service, FDN remains relevant and supported in 5G devices, demonstrating the enduring need for simple, device-enforced call restriction mechanisms.

Purpose & Motivation

FDN was created to address the need for granular, user-controlled call restriction directly on the mobile device. Prior to its introduction, call control was primarily a network operator service (like call barring) or a simple handset feature like keypad lock, which were either too broad or easily circumvented. There was a growing demand, particularly from corporate and parental users, for a method to restrict device usage to a specific set of numbers to control costs and ensure appropriate use. FDN solved this by leveraging the security and portability of the SIM card to store a whitelist, making the restriction persistent and tied to the subscriber identity rather than a specific, potentially replaceable, handset.

The motivation stemmed from the commercialization of mobile phones and the rise of premium-rate services and international roaming, where unintended calls could lead to significant expenses. For businesses, providing employees with mobile phones posed a risk of unauthorized personal or international calls. FDN provided a straightforward administrative tool to mitigate this financial risk. For the consumer market, it offered parents peace of mind by allowing them to configure a phone for a child with only essential contacts, enhancing safety. The technology leverages the existing USIM security model (PIN2) to protect the configuration, ensuring that only authorized persons (e.g., a parent or IT administrator) could modify the allowed number list.

Its continued inclusion through every 3GPP release up to Rel-19 underscores its utility as a foundational telephony service. While network-based policies and mobile device management (MDM) solutions have evolved to offer more sophisticated controls, FDN remains a simple, standardized, and universally supported fallback mechanism that works without network dependency. It addresses the core problem of basic outgoing call authorization in a way that is interoperable across networks and devices, a principle that remains valuable even in advanced 5G systems.