To understand call flows, you must first understand the nodes (hardware/software elements) that make up the EPC.

The EPC is comprised of several key logical nodes, each performing specific functions to maintain connectivity and service quality:

The interface to the outside world (e.g., the Internet). It assigns IP addresses to devices and enforces Quality of Service (QoS).

The "Attach" procedure is the most critical call flow, as it’s how a device (UE) joins the network to get "always-on" IP connectivity. LTE EPC is the Core Network of LTE networks. - YateBTS