Unlike previous 3GPP wireless technologies, LTE has no Circuit Switched (CS) bearer to support voice, so carrying voice over LTE requires a migration to a Voice over IP (VoIP) solution. Until this migration occurs, LTE-capable handsets need to revert to 2G or 3G for voice calls, which can reduce quality or even suspend Packed Switched (PS) services.

The GSMA’s IP Multimedia Subsystem (IMS) Profile for Voice and SMS document, commonly referenced as Voice over LTE (VoLTE), defines the mandatory set of features that the mobile device and network are required to implement in order to guarantee an interoperable, high quality IMS-based telephony service over LTE.

The course focuses on VoLTE services, underpaying IMS architecture, basic procedures and their impact on existing operator infrastructure.

The intermediate solutions like CSFB and SMSoSGs are also explained as they can be used concurrently with VoLTE to support roaming subscribers and emergency calls as long as the operator is not ready to move those service to VoLTE. 

Link: http://goo.gl/ZNMXdD