Among the different use cases related to voice services, the high quality audio conferencing scenario over heterogeneous networks and with different terminals, described more in details below, is a relevant scenario considered for this study item for the following reasons:
It is one of the most demanding for enhanced quality of service and high voice quality because of longer duration of conference calls.
It requires interoperability and flexibility since audio conferencing service may interconnect different users connected from different access networks and devices.
The use case describes the scenario of a multipoint audio conferencing session with N participants over EPS.
The participants are connected over heterogeneous access networks that include 3GPP access (e.g. E-UTRAN) and non-3GPP access networks, like WiMAX, WLAN, cdma2000®, PSTN…, with different QoS (bit rate, delay, packet loss, and jitter). They are also supposed to be in a situation of mobility between different access networks. They are participating using different types of terminals and in different environments (home, office, car, train…). Such terminals have different capabilities in terms of:
Supported codecs: new terminals support the EPS's codec(s) but old terminals can still participate in the conference even with only 3GPP pre-EPS codec(s).
The following examples are some possibilities illustrating the diversity of the participation environment conditions:
User A with PC terminal connected to WLAN.
User B with a laptop terminal working outside and connected to its home network (e.g. non 3GPP defined).
User C with a 3G/EPS hand held terminal in car connected through the LTE access network. This participant is supposed to continue seamlessly the participation in the conference call when arriving to his office but using e.g. WLAN connection.
User D with 2G or 3G terminal in train connected through GSM/GPRS/EDGE or UMTS with noisy background.
A.2 Two-party communication Word‑p. 13
The most typical and most important use case, where the end-user has high expectations regarding the perceived audio quality, is conversational voice communication between two parties (e.g. Multimedia telephony).
Within the scope of 3GPP systems, the most likely case is that at least one of the involved terminals is a mobile terminal. Mobile terminals can in many cases be assumed to be used in mobile environments with certain ambient noise level. Also, it is likely that the mobile terminals will be used in hand-held mode using the built-in microphones and loudspeakers, or with a monaural mobile handsfree set. It is however notable that there is a technology trend significantly enhancing the acoustic front-ends of mobile terminals. It can hence be assumed that there are an increasing number of cases in which the mobile terminal provides fullband and stereo (or multi-channel) capabilities. Furthermore, such cases can be regarded typical for fixed line, PC, or IPTV terminals. To optimize the end-user experience, during the codec negotiation a wideband codec should be prioritized over a narrowband codec. The network operator policy can override the codec negotiation.
Today, 3GPP systems have a subscriber base counting in billions. The introduction of EPS can hence only be gradual and interoperation with pre-Rel-10 systems and terminal equipment as well with Rel-10 equipment not supporting EPS is a likely use case still for many years to come.
Although multiple methods to achieve interoperation may exist, the selected method of interoperation should ensure the highest possible quality for the end user relative to other possible methods of interoperation.
By their nature mobile terminals are likely to move during a session. Together with the fact that EPS will not have 100% coverage from its first roll-out this means that there is a high likelihood for handovers between cells with EPS support and cells without. Cells without are cells with only CS or legacy pre-Rel-10 PS access type. This scenario is likely to happen both with mobiles participating in two-party as well as in multi-party communication use cases.
For mobile terminals which camps in EPS the services continuity as described in TS 22.278
These are supplementary services defined in 
. The call hold service allows a served mobile subscriber (subscriber A) to interrupt communication on an existing active call (with subscriber B), to place another call (with subscriber C) and, if desired, to re-establish the original communication.
The ECT supplementary service enables the served mobile subscriber (subscriber A) who has two calls, each of which can be an incoming or outgoing call, to connect the other parties in the two calls (subscribers B and C) and release the served mobile subscribers own connection.
In both of these supplementary services it is possible scenario that one of the calls established with subscriber A is an enhanced EPS voice service while the other is a legacy 3GPP wideband or narrowband voice service.
A.6 Voice-mail service Word‑p. 14
A typical use case of voice mail service is that a first user A establishes a call to a voice-mail server and leaves a message for some other user B. Later user B connects to the server and listens to the message. One likely case is that the terminals of both users are of different capabilities.
An increasing number of conversational services between two or multiple parties include access to media servers which put requirements on music and mixed content (speech + music) quality. This use case involves server-based media access like:
Calling participants of a conference call are listening music on hold waiting for the conference call organizer to open the bridge.
User is provided with a mixed content (speech + music) message (informative or advertising) from a multimedia server while waiting for the end user to answer.
At present, music while on hold offers little more to the user than providing an indication that the call has not dropped. EPS is an opportunity to provide music on hold, of comparable quality to multimedia streaming, to improve user experience.