Learn Spanish (Spain): ¿Está Pablo? ¿De parte de quién?

On the phone, simply asking if someone "is" using the verb estar is the standard way to ask if they are there. You don't need to add ahí or allí (there) because estar already covers the idea of being present or available.

Why do we use estar here instead of ser?

The verb estar is used for location and physical presence. Since you are asking if Pablo is physically located at the place you are calling, you must use estar.

How does ¿De parte de quién? translate to "Who's calling?"

Literally, it means "On the part of whom?" or "On whose behalf?". It is a standard, polite set phrase used in Spain to ask who is on the phone. You will hear this all the time from receptionists or family members answering a call.

Can I just say ¿Quién llama? instead?

Yes, ¿Quién llama? literally means "Who is calling?" and is perfectly correct. However, ¿De parte de quién? is a much more natural and customary phone response in Spain, so it is a great phrase to memorize.