What's the best response to creepy late-night phone calls?

Suppose you are a young woman living alone in a very safe neighborhood in a city that has been repeatedly deemed the “Safest City in America.” Safety has never been a concern for you, and it is a running joke with your friends that you can break into your apartment in less than thirty seconds.

Late one night, starting around 1:00am, you are awoken by a restricted number calling your phone. Concerned, you answer. The person on the other line makes some creepy-sounding whispers, but you can’t make anything out. You repeatedly ask the person to speak up and when s/he doesn’t, you hang up. The phone calls continue over and over again until you turn off your phone.

What is a reasonable amount of precaution to take before going back to sleep? Would you forget about it and do nothing? Call a friend, call the cops, leave your house? Remember, it is now well after 1:00am…but your house is very easy to get into…