5 Easy Steps: How 4G LTE & 5G NSA Call Flow Works

Modern mobile networks separate signaling from user traffic. Signaling authenticates the subscriber, creates bearers, applies policy, and locates the called party. The user plane then carries voice packets or application data. In LTE, the eNodeB connects the handset to the Evolved Packet Core (EPC); in 5G Non-Standalone (NSA), LTE remains the control-plane anchor while a 5G NR gNodeB adds radio capacity for the user plane.

A 4G voice call is normally VoLTE: an end-to-end IP service controlled by the IP Multimedia Subsystem (IMS). Before dialing, the handset performs LTE attach, receives a default EPS bearer through the MME, SGW, and PGW, discovers the P-CSCF, and registers its IMS identity using SIP. When the user calls, SIP INVITE signaling negotiates the destination, codec, and media parameters. Policy control then creates a dedicated bearer with conversational QoS—commonly QCI 1 for RTP voice—while IMS signaling commonly uses QCI 5.

In 5G NSA Option 3/3x, the device is dual-connected. The LTE eNodeB is the Master Node and maintains RRC/NAS signaling toward the EPC; the NR gNodeB is a Secondary Node added after LTE connectivity is established. Because this architecture still uses EPC and IMS, an ordinary voice call remains VoLTE over the LTE anchor. NR may continue carrying other data in parallel, depending on device and network capability. If VoLTE is unavailable, the network may use CS Fallback to 3G/2G where legacy service still exists.

For mobile data, the handset first establishes radio connectivity and sends an attach request. The MME authenticates the SIM with the HSS, selects gateways, and creates a default EPS bearer. The PGW assigns an IP address, while the PCRF supplies policy and charging rules. Application packets then travel UE → eNodeB → SGW → PGW → internet. With 5G NSA, the same EPC session is retained, but selected user-plane traffic can be split or moved through the NR gNodeB for higher throughput.

The key operational insight is simple: successful service requires every layer to work in sequence. RF coverage and RRC access come first, followed by NAS authentication, bearer creation, IMS registration for voice, and finally SIP call setup or application traffic. Engineers therefore correlate RRC, S1AP/NAS, GTP-C, Diameter, and SIP traces when troubleshooting setup failures, one-way audio, drops, or slow data.