What is the difference between Cellular calls and VoIP calls in LTE. As both use IP packets to send data to EPC. Without internet VoIP calls doesn't work but cellular calls work. Am I missing something?
VoLTE calls are the true operator calls, made directly on the phone dialer and whose audio packets go through your carrier's 4G IMS network. VoLTE calls are a type of VoIP call (such as VoLTE, VoWifi, VoNR). They use a dedicated EPS bearer that has controlled quality of service determined by your carrier. If your operator or your phone doesn't support VoLTE/IMS, your phone will fallback to 2G/3G to complete the call. This is not a VoIP call, but a CS (circuit-switched) call -- as 2G/3G are not PS (packet-switched) networks.
So the "cellular calls" in LTE are also VoIP calls -- they're just not OTT (over-the-top) VoIP calls.
These OTT VoIP calls (WhatsApp/Skype/Zoom calls) are calls made by applications that can only use the default internet bearer, so they don't have guaranteed quality of service for that data bearer. Packet delay (latency), packet loss, priority and bitrate parameters are completely different for that bearer.
These OTT VoIP calls lack very important features such as CS fallback, call continuity (SRVCC), supplementary services and most importantly emergency services (911).