The phrase “who is this?” in Spanish is usually:

  • ¿Quién es? – general, very common in most situations.
  • ¿Quién habla? – literally “who is speaking?”, used especially on the phone.
  • ¿Quién es este? / ¿Quién es esta? – when you’re pointing to or asking about a specific man (este) or woman (esta) nearby.

All of these are natural; context decides which sounds best.