Someone asked a question earlier about saying "Bueno" on the phone
Someone earlier asked we as mexicans say Bueno on the phone instead of Hola...? i have a answer.
¡Bueno!: Una de las maneras de atender el teléfono, en vez de nuestro HOLA
It is the short way of saying "Buenos dias!" It can be used instead of "Hola," but either way is correct.
Hope this helps!
When I was in Mexico I also heard "bueno" instead of "hola" when someone answered the phone.