Connected Line identification Restriction

Description

Connected Line identification Restriction (COLR) is a standardized supplementary service within the 3GPP framework that enables a called subscriber to prevent their phone number (MSISDN) from being displayed to the calling party during a call setup. This service operates within the core network, specifically interacting with call control functions in the Mobile Switching Center (MSC) for circuit-switched calls and the IP Multimedia Subsystem (IMS) for packet-switched VoIP calls. When a call is initiated, the network checks the called party's subscription profile to determine if COLR is active. If enabled, the network suppresses the transmission of the connected line's identity (CLI) in signaling messages, such as the ISDN User Part (ISUP) or Session Initiation Protocol (SIP) headers, ensuring the caller receives no indication of the actual number answered.

Architecturally, COLR relies on the Home Location Register (HLR) or Home Subscriber Server (HSS) to store the subscriber's service profile, including COLR settings. During call establishment, the serving MSC or IMS Call Session Control Function (CSCF) queries this profile. If COLR is invoked, the network modifies the call signaling to replace or omit the connected line's identity, often substituting it with a generic indicator like 'restricted' or leaving the field blank. This process is transparent to the called party and requires no manual activation per call, as it is a subscription-based feature managed by the network operator.

COLR interacts closely with other supplementary services, such as Calling Line Identification Presentation (CLIP) and Calling Line Identification Restriction (CLIR), forming a comprehensive set of privacy and identification controls. In IMS networks, COLR is implemented using SIP mechanisms, where the P-Asserted-Identity header may be altered or removed based on the called party's preferences. The service is crucial for scenarios where the called party wishes to maintain anonymity, such as in helplines, business contact centers, or personal privacy concerns, ensuring that the caller cannot retrieve the connected number through network signaling.

From a protocol perspective, COLR is defined in 3GPP specifications like TS 23.018 for basic call handling and TS 29.864 for IMS enhancements. It involves specific information elements in call control messages, such as the Connected Line Identity Parameter in ISUP or the SIP Privacy header. The network must handle conflicts between COLR and other services, like explicit override features for emergency services, ensuring regulatory compliance. Overall, COLR is a foundational privacy feature that integrates seamlessly into mobile and fixed-line networks, protecting subscriber identity without disrupting call connectivity.

Purpose & Motivation

COLR was introduced to address growing privacy concerns in telecommunications, allowing called parties to control the disclosure of their phone numbers. Prior to its standardization, callers could often view the connected line's identity through network signaling, which posed risks for individuals and organizations needing confidentiality, such as victims of harassment, confidential business lines, or public service hotlines. The service provides a standardized mechanism to restrict this information, ensuring consistent privacy protection across different network operators and regions.

The creation of COLR in 3GPP R99 was motivated by the need to complement existing identification services like CLIP and CLIR, forming a balanced ecosystem where both calling and called parties have control over identity presentation. It solved the limitation where called parties had no way to prevent their number from being revealed, even if they restricted outgoing caller ID. This was particularly important with the rise of mobile communications, where personal numbers are often linked to individuals, increasing the potential for misuse if exposed.

Historically, COLR evolved from fixed-line telephony features, adapted for mobile networks to support subscriber mobility and roaming. It addresses legal and regulatory requirements in many jurisdictions that mandate privacy protections, ensuring networks can comply with data protection laws. By providing a network-based solution, COLR eliminates the need for end-users to rely on external devices or manual tricks to hide their numbers, offering a reliable and standardized approach that works seamlessly across international boundaries and network technologies.

Key Features

• Prevents display of called party's MSISDN to calling party
• Subscription-based service managed via HLR/HSS profiles
• Integrates with both circuit-switched (MSC) and packet-switched (IMS) networks
• Uses signaling modifications in ISUP or SIP protocols
• Works transparently without called party intervention per call
• Interoperates with CLIP and CLIR for comprehensive identity management

Evolution Across Releases

Defining Specifications