Calling Line Identification Restriction

Description

Calling Line Identification Restriction (CLIR) is a standardized supplementary service within 3GPP networks that enables a calling subscriber to restrict the presentation of their calling line identity (CLI) to the called party. The service operates by modifying the signaling messages exchanged during call establishment, specifically within the Initial Address Message (IAM) in ISUP or the Setup message in ISDN/BICC protocols. When CLIR is invoked, the network either omits the calling party number information element or marks it as 'presentation restricted' according to the subscriber's permanent service subscription or temporary invocation via a prefix (e.g., *67).

The technical implementation involves multiple network elements. The originating Mobile Switching Center (MSC) or Gateway MSC (GMSC) evaluates the calling subscriber's service profile, which is stored in the Home Location Register (HLR) or Home Subscriber Server (HSS). Based on the subscriber's CLIR subscription setting—which can be 'permanent' (applies to all calls) or 'default' (with override capability)—the MSC manipulates the 'Presentation Indicator' field within the calling party number information element. For permanent restriction, the CLI is consistently withheld. For default settings, the subscriber can use a prefix to invoke restriction per call or a suffix to allow presentation per call.

In the core network signaling, CLIR interacts with the Calling Line Identification Presentation (CLIP) service at the terminating side. When a call with restricted CLI arrives at the terminating MSC, the presentation indicator is examined. If marked as 'restricted,' the terminating network does not deliver the CLI to the called party's terminal, or it may display 'private number,' 'anonymous,' or similar. The service also works in conjunction with other supplementary services like Connected Line Identification Presentation (COLP) and must handle various scenarios including international calls, where restrictions might be governed by national regulations.

The service architecture ensures backward compatibility and interworking with legacy networks (PSTN, ISDN). In all-IP networks (IMS), CLIR is implemented using SIP headers, specifically the P-Asserted-Identity header with privacy indicators. The Serving-Call Session Control Function (S-CSCF) applies the subscriber's profile to set the appropriate privacy headers. CLIR is a fundamental privacy feature that operates transparently across circuit-switched and packet-switched domains, maintaining user privacy while ensuring call completion.

Purpose & Motivation

CLIR was created to address growing privacy concerns in telecommunications, allowing individuals and organizations to control the disclosure of their phone numbers. Before its standardization, callers had no standardized method to prevent their number from being displayed, which could lead to unwanted contact, harassment, or breach of confidentiality in sensitive situations (e.g., healthcare, law enforcement, domestic violence cases). The service provides a crucial balance between the called party's right to know who is calling and the calling party's right to privacy.

Historically, caller ID services (CLIP) became popular in the 1980s and 1990s, creating demand for a corresponding restriction capability. 3GPP standardized CLIR in Release 99 to ensure consistent implementation across GSM, UMTS, and later LTE/5G networks, replacing proprietary solutions. The service solves the problem of involuntary disclosure of personal information, particularly important with the rise of mobile phones where numbers are more personally identifiable than landline numbers.

CLIR also addresses business needs where companies want to protect their internal extension numbers or call center agents' direct lines. It enables compliance with privacy regulations (e.g., GDPR) by providing technical means to withhold personal data. The service's flexible implementation—allowing both permanent subscription and per-call invocation—caters to diverse user preferences while maintaining network efficiency and interoperability across different operators and countries with varying privacy laws.