Plugged my earphone into one of my phones. While I was picking things to listen to, I suddenly heard a voice said, "Hello."

I immediately removed the earphone from my ears. Quickly checked if I had somehow accidentally dialled someone (nothing). Haven't even started playing anything yet. Then I restarted the phone.

No, it's not haunted!

I went through the audio files I have on that phone and one of it was from Whatsapp. It sounded vaguely like the voice I heard. I really think that was it. 

Weird. 

How did it end up playing when I plug in the earphone?

Of course, I can think of some logical situations (think connecting your phone via bluetooth to the car and it immediately plays whatever's recent on your Spotify app) but whoa... I haven't use the Music app on the phone for so long. Couldn't see why it would be triggered(?) when the earphone is plugged in. 

...won't think too much about it.