The Attach Procedure registers the UE with the network to receive services. Here is the step-by-step control plane flow:

The MME, eNodeB, and S-GW establish radio bearers and S1-U tunnels for user data transfer. 3.3 Tracking Area Update (TAU) - Mobility

Here are some common EPC call flows:

The UE sends an Attach Request to the MME via the eNodeB.