Customized Ringing Signal

Description

The Customized Ringing Signal (CRS) service is an operator-specific supplementary service defined in 3GPP TS 22.183, with security aspects addressed in TS 33.106. It provides an indication to the called party as an incoming communication indication during the establishment of a communication, where that indication is customized by the calling party or the called party. The customized content may take many forms, including a picture, a piece of recorded or composed music, greeting words, voice, an advertisement, or video. Unlike the Customized Alerting Tone (CAT) service, which customizes what the caller hears, CRS customizes what the called party experiences when receiving an incoming communication.

Architecturally, CRS is realized as an application-layer service that interacts with the call control layer in both the CS and IMS domains. When an incoming communication is directed to a CRS subscriber, the terminating network detects the CRS subscription and, instead of presenting the default ringing indication, retrieves and presents the customized content to the called party. This requires the network to identify the appropriate content based on the calling party identity, the called party's preferences, or other context, and to deliver that content to the called terminal in a timely manner during the alerting phase. The content may be stored in a CRS application server and streamed or downloaded to the terminal, or it may be pre-provisioned on the terminal and selected by a network-provided identifier.

Key components of a CRS deployment include the CRS application server that manages content storage and selection logic, the subscriber profile in the HSS/HLR that indicates CRS subscription and preferences, the call control functions (MSC server, IMS CSCF) that trigger CRS invocation, and the terminal capability to render the customized content. Because CRS content can include rich media such as pictures and video, terminal capabilities and codec support become important considerations. The service must also handle the case where the called party's terminal cannot render the selected content, falling back to a default ringing indication.

Security and privacy are significant concerns for CRS, which is why TS 33.106 addresses security aspects. The service must ensure that customized content is delivered only to authorized parties, that content is not spoofed or tampered with, and that the privacy of both calling and called parties is protected. Content licensing and intellectual property rights also come into play when subscribers select commercial music or video as their ringing signal.

CRS has been standardized since Rel-9 and has remained stable through Rel-19. The definition was refined in Rel-10 to clarify that CRS is an indication to the called party during communication establishment, rather than simply a "ringing signal," reflecting a more precise understanding of the service's role in the call flow. In modern IMS and VoLTE/VoNR deployments, CRS is implemented through SIP-based application servers and can leverage rich media capabilities of smartphones to deliver a compelling user experience.

Purpose & Motivation

The CRS service exists to enrich the incoming communication experience for the called party, providing a personalization and branding channel that complements the CAT service. Before CRS, the incoming call indication was a generic ringtone or vibration with no customization potential beyond what the terminal itself could provide locally. CRS moves this customization into the network, enabling content that is selected by the calling party or the called party and delivered as part of the communication establishment.

By standardizing CRS, 3GPP addressed the need for interoperable, operator-controlled customization of the ringing experience. The service allows operators to offer differentiated packages, enables subscribers to express identity or mood through their ringing signals, and provides a platform for advertising and enterprise branding. It also solves the problem of content portability: because the content is managed in the network, it can be delivered regardless of the terminal model, subject to rendering capabilities.

The Rel-10 wording change from "A Ringing Signal that is customized..." to "An indication to the called party as an incoming communication indication during the establishment of a communication that is customized..." reflects a maturation of the service concept, clarifying that CRS is not merely a replacement ringtone but a broader incoming communication indication that occurs during call establishment. This precision helps guide implementation and interoperability across diverse network and terminal environments.