the radio interface. Hence, the KPIs for voice services in mobile networks cover all KPIs for fixed VoIP as well as additional ones that refer to mobile networks only. The first standardized VoIP in mobile networks is Voice over LTE (VoLTE), defined for 4G mobile networks (i.e. LTE/LTE-Advanced/LTE-Advanced-Pro), which started with deployments in the late 2010s (toward 2020) and will continue in the 2020s (i.e. in the 5G era). However, the transition of PSTN/ISDN to NGN-based VoIP first happened in fixed access networks due to mobility management issues in mobile networks (that is, the handovers, which require strict synchronization to accommodate mobile voice services, and that is more complex in all-IP environment than in TDM, e.g. TDM by default has synchronization end-to-end as well as there is no buffering of data in the network nodes - something that adds additional packet delay and jitter to the VoIP). Considering VoLTE, the relevant KPIs are specified in ITU-T G.1028 [8]. The VoLTE is provided over a managed IP network, SO it is also referred to as a "managed" VoIP ser- vice (which is provided with its prioritization over the other IP traffic, such as Internet traffic). That is in contrast to OTT VoIP applications/services (Skype, Viber, WhatsApp, etc.) which use the public Internet access that is network neutral and it has no prioriti- zation over the rest of the Internet traffic (video, Web, etc.), although telecom operators may apply traffic management techniques to the aggregate Internet traffic (with the use of DPI) with the aim of offering appropriate QoS to different types of public Inter- net traffic. Additionally, VoLTE uses the NGN control functionalities based on SIP and Diameter [9], used in IMS (IP multimedia subsystem) in NGN deployments in fixed and mobile networks. In 4G mobile networks (LTE/LTE-Advanced/LTE-Advanced-Pro), the quality classi- fication identifiers (QCIs) that can be used for VoLTE services are given in Table 7.4. However, the KPI target values are dependent whether the VoIP