Connected Line identification Presentation

Description

Connected Line identification Presentation (COLP) is a standardized supplementary service defined within the 3GPP framework, operating as part of the Intelligent Network (IN) and Core Network service layer. It functions by enabling the network to deliver the directory number (or other identification) of the connected party—the party who ultimately answers the call—back to the originating calling party. This process occurs after the call setup is complete and the connection is established, providing a verification mechanism for the caller. The service is typically invoked during call establishment and relies on signaling protocols like ISDN User Part (ISUP) in circuit-switched domains or Session Initiation Protocol (SIP) in IP Multimedia Subsystem (IMS) networks to transport the connected line identification information.

Architecturally, COLP interacts with several network elements. In traditional circuit-switched mobile networks, it involves the Mobile Switching Center (MSC) and Home Location Register (HLR). The HLR stores the subscriber's service profile, including COLP subscription data. When a call is routed, the serving MSC of the called party retrieves this profile and, if COLP is active, includes the connected party's number in the backward call setup messages (e.g., in the ISUP Answer or Connect messages). In IMS-based networks, COLP is implemented using SIP methods and headers, such as the P-Asserted-Identity header, which conveys the verified identity of the connected user within SIP 200 OK responses to the INVITE request.

The service's operation is tightly coupled with privacy and subscription settings. A subscriber must be provisioned for COLP, and the presentation of the connected number can be restricted by the connected party using the Connected Line Identification Restriction (COLR) service. The network performs a check: if the connected party has COLR active, the COLP service will not present the number to the caller, instead providing an indication that presentation is restricted. This interplay ensures user privacy controls are respected. From the caller's perspective, the presented connected number appears on their terminal's display, allowing them to confirm they are connected to the intended party, which is vital for preventing fraud or misrouting in scenarios like call forwarding or operator services.

COLP's role extends beyond simple number display; it is a foundational element for service transparency and trust. In advanced telephony services like call diversion, call transfer, or business applications where calls are answered by an automated system or agent, COLP provides the caller with assurance about the actual answering endpoint. Its implementation is mandatory for network operators to support certain regulatory requirements and is a key differentiator for premium telephony services. The service works in tandem with Calling Line Identification Presentation (CLIP), creating a bidirectional identification framework that enhances the overall user experience and security of telecommunication services.

Purpose & Motivation

COLP was created to address the asymmetry in caller identification services. While CLIP allowed the called party to see who was calling, there was no standardized mechanism for the calling party to verify the identity of the party who actually answered the call. This gap became problematic with the proliferation of call forwarding, voice mail systems, and operator services, where a caller might be connected to a different number than the one dialed. Without COLP, callers could be misled, leading to potential fraud, confusion, or lack of trust in telecommunication services, especially in business and emergency contexts.

The service was introduced in 3GPP Release 99, building upon existing ISDN standards, to provide a complete identification framework. It solves the problem of call destination verification, ensuring that users have transparency about the connected line. This is particularly important for mobile networks where features like call forwarding on busy or no reply are common. COLP empowers callers by confirming the answering party's identity, which enhances security against spoofing or malicious redirection and supports regulatory requirements for call traceability and consumer protection.

Historically, prior to COLP, some proprietary solutions existed, but they lacked interoperability across different network operators and equipment vendors. The standardization within 3GPP ensured a uniform approach, enabling seamless service delivery in multi-vendor, multi-operator environments. By integrating COLP into the core network service architecture, 3GPP addressed the limitations of earlier systems that could not reliably provide connected party identification in complex call routing scenarios, thereby increasing the reliability and trustworthiness of telephony services across GSM, UMTS, and subsequent generations.

Key Features

• Presents the directory number of the connected (answering) party to the calling party
• Operates as a supplementary service integrated with call control signaling
• Interworks with Connected Line Identification Restriction (COLR) for privacy management
• Supported in both circuit-switched (ISUP) and IMS (SIP) network domains
• Requires subscription provisioning in the HLR or user profile
• Provides verification against call forwarding or diversion scenarios

Evolution Across Releases

Defining Specifications