What is the most universally-used greeting when answering the phone (i.e. way to say, "Hello?")? Are there any circumstances where the greeting would be different (for example, when answering a phone call where you already know who is calling)? What regional differences are there in this area from country to country?
Complementing Alenanno's answer, I summarized this Wordreference thread :
- ¿Bueno?: Mexico
- ¿Sí?: Mexico, Spain
- ¿Aló?: Colombia, Chile, Peru, Costa Rica, Venezuela
- Hola: Argentina
- ¿Diga?: Spain, Argentina
- ¿Dígame?: Spain
- ¿Oigo?, ¿Dígame?: Cuba
I found a page with some polite language on the phone in Spanish.
The "¿Bueno?" you see at the beginning is maybe understood by any Spanish speaker, but it seems it's mostly used in Mexico. In other Latin American countries they say "¿Aló?". In Spain they use "¿Dígame?" or ¿Diga?".