Back in November, f(x) members Krystal and Luna had a lovely duet titled You're My Destiny. Now the talented duo are paired up once again for the Cinderella's Sister OST (Soundtrack). The track is titled ???? (Calling Out) and it's a very lovely track. Many people think Krystal's voice sounds a lot like her sister (SNSD's Jessica) and I noticed it a lot more in this particular song. Thanks to jenpooable for the tip and check out the track below.