On the phone in Spanish - Who's calling?
Different Spanish speaking countries have different ways of answering the phone. For example, in Spain it can be "diga", "digame" or "¿sí?", in Mexico it can be "bueno", and in Colombia it can be "¿aló?". I say "can be" because there might be other ways of answering, depending on the situation.
The following are some ways to ask "Who's calling?". I've indicated whether they are "formal" or "informal" which in this situation means the difference between asking when you think that it might be a friend (informal) and when you don't have any idea who it is (formal).
1. ¿Quién llama? (formal)
2. ¿Quién es? (formal)
3. ¿Quién eres? (informal)
4. ¿Eres Juan / María? (informal meaning "Is that Juan / María?).
And if someone calls asking to speak to someone else, you can use the very formal "¿De parte de quién?" which translates literally as "On behalf of whom?" although it doesn't sound as heavy in Spanish.