INcoming Call

Description

INcoming Call (INC) is a core service concept within the 3GPP specifications, specifically detailed in TS 27.002. It represents the complete set of network procedures and signaling interactions initiated when a call is placed to a mobile subscriber (the called party). The INC process is not a single entity but a service description that encompasses the sequence from the point the call enters the mobile network to the point it is either answered, forwarded, or rejected. It defines the expected behavior of network elements like the Mobile Switching Center (MSC) or IP Multimedia Subsystem (IMS) nodes in response to this event.

The technical operation of an INC involves several key stages. First, the call routing function identifies the serving network and node (e.g., MSC or S-CSCF) for the called subscriber. The serving node then retrieves the subscriber's service profile, which includes crucial data like call forwarding settings, barring lists, and current registration status (e.g., idle, busy, not reachable). Based on this profile, the network executes service logic. This may involve sending a paging request to locate the mobile device, applying call forwarding unconditional (CFU) or on busy (CFB), or playing an announcement if the subscriber is barred from receiving calls.

In the context of the core network architecture, the INC procedures are integral to the Call Control (CC) and Mobility Management (MM) protocols. For Circuit-Switched (CS) calls, the MSC is the central actor, using ISUP or BICC signaling for trunk interconnection and MAP protocols to query the HLR for routing information. For IMS-based Voice over LTE (VoLTE) or Voice over NR (VoNR), the INC is handled by the S-CSCF, which acts as the service control point, executing initial Filter Criteria (iFC) from the subscriber's profile and orchestrating SIP signaling towards the User Equipment (UE). The INC definition ensures that regardless of the underlying transport (CS or PS), the service experience from the caller's perspective—ringing tones, forwarding, voicemail diversion—is consistent and reliable.

Purpose & Motivation

The purpose of standardizing the INcoming Call (INC) concept is to ensure interoperability and consistent user experience across multi-vendor networks and between different generations of mobile technology (e.g., GSM, UMTS, LTE, 5G). Before such standardization, proprietary implementations could lead to incompatible call handling behaviors, causing failed call setups, unexpected call forwarding, or inconsistent tone provision when roaming between networks. By defining INC in a technical specification, 3GPP provides a blueprint that equipment manufacturers and network operators must follow.

Historically, the concept originates from the need to abstract the complex call handling logic from the physical switching hardware. In early digital exchanges, call processing was tightly coupled to the switch's software. 3GPP's service-oriented approach, evident from GSM onwards, separated service logic (like handling an incoming call) from the connection control. This separation allowed for more flexible introduction of new services and smoother evolution from CS to IMS-based calling. The INC definition solves the fundamental problem of how a network should locate a mobile user, apply their subscribed services, and establish a bearer path, all while adhering to regulatory requirements like lawful interception and emergency call prioritization.

Its continued relevance through to Release 19 underscores that despite the transition to all-IP networks, the basic telephony service of receiving a call remains paramount. The INC procedures have evolved to incorporate IMS features like Session Border Controller (SBC) interactions, emergency call routing enhancements (e.g., for IoT devices), and integration with network slicing for mission-critical voice, but the core purpose—reliably terminating a call to a subscriber—remains unchanged.

Key Features

• Defines the end-to-end signaling flow for mobile-terminated call setup
• Incorporates subscriber service logic execution (e.g., call forwarding, barring)
• Integrates with mobility management procedures like paging and location updating
• Supports both Circuit-Switched (ISUP/BICC) and Packet-Switched (SIP) call control protocols
• Provides the framework for interaction with the HLR/HSS for subscriber data and routing information retrieval
• Enables consistent user experience through standardized call progress tones and announcements

Evolution Across Releases

Defining Specifications