Last Number Dialled

Description

Last Number Dialled (LND) is a basic but essential supplementary service in circuit-switched telephony networks, including GSM, UMTS, and legacy aspects of IMS. It is a user-facing feature managed by the mobile device (Mobile Station) in conjunction with network support. Functionally, the MS stores the last successfully dialled and established (or attempted) telephone number in its volatile or non-volatile memory. When the user activates the LND feature—often by pressing a 'redial' soft key, a dedicated hardware button, or a short code like *#—the MS retrieves this number and automatically initiates a new call setup procedure using it.

From a network architecture perspective, LND is primarily a UE-centric feature. The core network (MSC) is not directly responsible for storing or providing the last dialled number; its role is to process the new call setup request that the UE generates. However, the network must support the general call control procedures (detailed in 24.008 and other core specs) that allow the UE to originate a call. The service relies on the standard Mobile Originating (MO) call setup signaling flow. The UE populates the Called Party BCD Number information element in the SETUP message with the stored number and sends it to the network.

How it works is straightforward but involves coordination between the Man-Machine Interface (MMI) of the phone and its call control layer. The call control layer logs the destination number from a successfully initiated MO call. This logging typically occurs when the UE sends a SETUP message to the network. Some implementations may also store numbers from attempted calls that failed to connect. Upon redial activation, the MMI requests the call control layer to place a call to the stored number. The call control then proceeds with the standard authentication, ciphering, and call establishment procedures as if the user had manually keyed in the number. The feature's simplicity belies its utility, as it eliminates manual re-entry, reduces errors, and improves the user experience, especially for frequently contacted numbers or after a failed call attempt.

Purpose & Motivation

LND was created to address the need for user convenience and efficiency in early and subsequent cellular networks. In the era of physical keypads and before contact lists were sophisticated, manually dialling a long telephone number (especially international numbers) was time-consuming and prone to errors. If a call failed due to congestion, poor coverage, or a wrong number, the user would have to re-enter the entire sequence. The LND feature solved this by automating the re-dial process with a single action.

Its introduction in Rel-5, as part of the broader set of GSM/UMTS supplementary services, reflected the industry's focus on improving the basic voice service experience. It addressed a clear limitation of previous manual dialling approaches. While seemingly trivial, such features were critical for user adoption and satisfaction in competitive telecommunications markets. The motivation was to make the phone 'smarter' and more user-friendly, reducing the steps required for common actions.

Even as networks evolved towards VoIP and IMS-based calling, the redial function persisted as a staple of phone UI design, though its implementation became entirely handset-based. In the IMS context, the SIP UA client on the device performs a similar logging and redial function for SIP URIs or tel URIs. Therefore, LND represents an early and enduring example of a user-centric service designed to minimize effort and maximize the utility of the voice communication service.