This time it's the robocallers that let the phone ring once & hangup that you need to be careful of
DO NOT CALL THEM BACK is the main message the FCC is trying to get across
According to KTLA 5:
The scam works like this: A scammer places a robocall to a number and hangs up after one or two rings. They may call back several times. The idea is to get the caller to call the number back. When they do, the caller is prompted to pay long distance fees to connect the call, fees that are usually paid in part to the scammer.