1. Introduction

The feature and functionality requirements for SIP user agents (UAs) supporting business telephony applications differ greatly from basic SIP UAs, both in terms of services and end-user experience. In addition to basic SIP support [RFC3261], many of the services in a business environment require the support for SIP extensions such as REFER [RFC3515], SUBSCRIBE/NOTIFY [RFC6665], PUBLISH [RFC3903], the SIP Replaces [RFC3891], and Join [RFC3911] header fields, etc. Many of the popular business services have been documented in the SIP Service Examples [RFC5359].

This specification details a method for implementing a group telephony feature known variously in telephony as Bridged Line Appearance (BLA) or Multiple Line Appearances (MLA), one of the more popular advanced features expected of SIP IP telephony devices in a business environment. Other names for this feature include Shared Call/Line Appearance (SCA), Shared Call Status and Multiple Call Appearance (MCA). A variant of this feature is known as Single Line Extension.

This document looks at how this feature can be implemented using standard SIP [RFC3261] in conjunction with SIP events [RFC6665] and publication [RFC3903] (carrying the SIP dialog state event package [RFC4235]) for exchanging status among UAs.

In traditional telephony, the line is physical. A common scenario in telephony is for a number of business telephones to share a single or a small number of lines. The sharing or appearance of these lines between a number of phones is what gives this feature its name. A common scenario in SIP is for a number of business telephones to share a single or a small number of Address of Record (AOR) URIs.

In addition, an AOR can have multiple appearances on a single UA in terms of the user interface. The appearance number relates to the user interface for the telephone; typically, each appearance of an AOR has a visual display (lamp that can change color or blink or a screen icon) and a button (used to select the appearance) where each appearance number is associated with a different dialog to/from the AOR. The telephony concept of line appearance is still relevant to SIP due to the user interface considerations. It is important to keep the appearance number construct because:

1. Human users are used to the concept and will expect it in replacement systems (e.g., an overhead page announcement says "Joe pickup line 3").

2. It is a useful structure for user interface representation.

The purpose of the appearance number is to identify active calls to facilitate sharing between users (e.g., passing a call from one user to another). If a telephone has enough buttons/lamps, the appearance number could be the positional sequence number of the button. If not, it may still be desirable to present the call state, but the appearance number should be displayed so that users know which call, for example, is on hold on which key.

In this document, except for the usage scenarios in the next section, we will use the term "appearance" rather than "line appearance" since SIP does not have the concept of lines. Note that this does not mean that a conventional telephony user interface (lamps and buttons) must be used: implementations may use another metaphor as long as the appearance number is readily apparent to the user. Each AOR has a separate appearance numbering space. As a result, a given UA user interface may have multiple occurrences of the same appearance number, but they will be for different AORs.