Learn Spanish (Spain): Un momento, ¿de parte de quién?

While ¿quién llama? literally means "who is calling?", ¿de parte de quién? is the standard, polite way to ask who is on the phone in Spain. It translates directly to "on behalf of whom?" or "from whose part?".

How should I reply if someone asks me ¿de parte de quién? on the phone?

You can reply using the exact same structure by saying De parte de followed by your name. For example, De parte de David tells the person that David is calling. You can also simply say Soy David.

Why does quién have an accent mark here?

In Spanish, question words always carry a written accent mark when they are used to ask a question. Here, quién has an accent because it is asking "who?", which helps tell it apart from the non-question word quien used in normal statements.

Is the phrase de parte de only used for telephone calls?

No, it is a very useful everyday phrase meaning "on behalf of" or "from" a person. For example, if you are delivering a message or a present from a friend, you could say it is de parte de that friend.