I have an LG G2 and have experienced the same problem... a chime with descending notes before the call is dropped.
I have replaced the LG G2 several times with other G2s and still the same problem. It doesn't matter if I have advanced calling on or off. It doesn't matter if I have data on or off. Or Wi-Fi on and off.
I believe an app is turning on airplane mode... which causes the disconnect. If I try to call back the caller immediately after the disconnect, an error says I cannot call in airplane mode. I also am always on speaker... so I know disconnect is caused by me accidentally clicking a button.