Any discussion of IMS protocols must start with a dialogue describing the procedures being implemented. It is important to note that there is no “one size fits all” procedural flow; IMS in LTE offers a lot of flexibility to both network equipment manufacturers and network operators. Note that the processes described here are strictly from the UE’s point of view, without
discussion of the many intra-network procedures required to make the system work.
The processes involved in a Voice over LTE (VoLTE) call can provide a meaningful background and a fairly typical scenario. From the UE’s point of view the initial step is to “listen” for system information in the form of Master Information Blocks (MIBs) and System Information Blocks (SIBs). Once that information has been processed the UE can initiate its own
processes. These processes are outlined in the next section. The graphical depiction in Figure 1 is not meant to distinguish between multiple protocol layers; it is merely an intuitive impression of the required processes.

Important IMS Procedures are

1. PDN Connectivity (NAS Signaling)
2. Authentication
3. Bearer Setup and EPS Attach
4. P-CSCF Discovery
5. SIP Registration
6. Event Subscription
7. VoLTE Call

Source: IMS Procedures and Protocols: The LTE User Equipment Perspective by Spirent