Successful call

Description

In 3GPP terminology, a 'Successful call' is defined as a connection that reaches the communication or data transfer phase. For speech connections, this corresponds to the 'answered' state, meaning the called party has picked up and a conversation can take place. Any connection attempt that does not reach this phase is regarded as unsuccessful. This definition is fundamental to how operators measure network performance, quality of service, and user experience.

The concept is used across multiple 3GPP specifications, including TS 32.240 (Charging architecture and principles), TS 32.250 (Charging management for circuit-switched domain), TS 32.272 (Charging management for IMS), TS 32.273 (Charging management for MMS), TS 32.276 (Charging management for location services), TS 32.278 (Charging management for MBMS), and TS 32.849 (Study on charging for 5G). These specifications rely on the definition to determine when a call is billable and how to classify call attempts for statistical and charging purposes.

From a charging perspective, a successful call typically triggers the generation of a charging record (CDR) that includes the duration of the communication phase. Unsuccessful attempts may generate different types of records or no record at all, depending on the charging model. This distinction is crucial for accurate billing and for preventing revenue leakage. It also affects key performance indicators (KPIs) such as call setup success rate (CSSR) and call completion rate, which are used to assess network health and customer satisfaction.

The definition is technology-agnostic, applying to circuit-switched (CS) calls, IMS-based voice over LTE (VoLTE) calls, and other real-time communication services. In packet-switched and IMS domains, the 'answered' state is typically signaled via SIP 200 OK messages, while in CS networks it is indicated by the connect message. The underlying principle remains the same: a call is successful only when the user can actually communicate.

Over the releases, the definition has remained consistent, reflecting its fundamental nature. However, its application has expanded with new services such as VoLTE, VoNR, and mission-critical push-to-talk (MCPTT), where the communication phase may be established through different signaling procedures. The term is also used in quality of experience (QoE) frameworks, where successful call rate is a key indicator of service quality.

Purpose & Motivation

The definition of a 'Successful call' exists to provide a clear, unambiguous criterion for determining when a call attempt should be considered successful. This is essential for charging, performance measurement, and service level agreement (SLA) monitoring. Without a standardized definition, different network elements or operators might classify call outcomes differently, leading to inconsistencies in billing, statistics, and regulatory reporting.

Historically, as telecommunication networks evolved from simple circuit-switched voice to complex multi-service architectures, the need for a common understanding of call success became more critical. Early networks might have considered a call successful upon seizure of resources, but this did not reflect the user experience. The 3GPP definition focuses on the communication phase, aligning technical measurements with actual service delivery. This helps operators identify real issues in network performance and ensures that charging is fair and accurate.

The definition also supports inter-operator settlement and roaming. When a subscriber roams, the visited network and home network must agree on which calls are billable. A consistent definition of successful call ensures that both parties use the same criteria, reducing disputes and simplifying settlement processes. In the context of new services like VoLTE and 5G, the same principle applies, ensuring that the definition remains relevant across generations.