Voice services are the main communications services provided by carriers. VoLTE provides a QoS-guaranteed VoIP solution and is becoming an inevitable trend in the development of wireless and core network technologies. With the extensive deployment of LTE networks, VoLTE has become a preferred voice solution for carriers. 

Mobile voice services have developed from using time division multiplexing (TDM) to IP and from using traditional switches to softswitches, and will evolve to mobile broadband voice services. GSM or UMTS wireless networks are evolving towards LTE networks, while the CS core network is evolving towards the IP multimedia subsystem (IMS). Over the past few years, there have been multiple viewpoints regarding LTE voice technologies and evolutions. As a result, the following four solutions have been recognized for their commercial applications: 

The first solution is Circuit Switched Fallback (CSFB). LTE only provides data services. When a voice call is initiated or received, the service falls back to the CS domain. Carriers do not need to deploy the IMS but must simply upgrade the mobile switching center (MSC). This solution can quickly provide voice services but the call setup duration is long. Therefore, CSFB can either be used as an interim solution before the IMS deployment or be used to provide voice services for roaming LTE subscribers when the IMS is not deployed in a visited network or IMS roaming protocol is not available. 

The second solution is Simultaneous Voice and LTE (SVLTE), namely its dual standby mode. Mobile phones simultaneously work with LTE providing data services, and CS providing voice services. This solution has no network special requirements and does not need to deploy the IMS. However, it is mobile phone-based, and requires high costs and consumes a lot of mobile phone battery. Currently, mobile phones using the CDMA 1x and LTE dual standby mode have been an interim solution before the IMS deployment. 

The third solution is over the top (OTT), such as using Skype to provide LTE voice services. With high bandwidth, low delay, always online, and all-IP features, LTE has laid a solid foundation for the development of OTT. Voice services continue to be the main source of carriers' revenues. Therefore, using the OTT to provide all LTE voice services is not widely accepted in the telecommunications industry. Carriers may have certain advantages in deploying LTE voice services over OTT, such as existing subscriber number resources, interworking based on protocols, QoS assurance capability, CS handover and roaming, and binding policies of mobile broadband packages. In the future, OTT may be increasingly used for voice services, especially for toll voice services. However, voice services provided by carriers will still be the mainstream in the long term. 

The fourth solution is IMS-based VoLTE. IMS supports the access of diversified multimedia services and is becoming a part of core network architecture in an all-IP era. Having matured significantly in recent years, IMS has become a leading choice for fixed network reconstruction of voice over broadband (VoBB) and public switched telephone network (PSTN). 3rd Generation Partnership Project (3GPP) and Global System for Mobile Communications Association (GSMA) have established IMS as a standard architecture for mobile voice services. CSFB and SVLTE solutions can provide voice services based on CS domains, however with certain limitations. Therefore, they are called pre-VoLTE solutions and can only be used as interim solutions but not mainstream solutions. In addition, only IMS can provide the QoS-guaranteed VoIP solution based on LTE networks. The IMS-based VoLTE is the inevitable result of technological development in radio networks and core networks.