llamar with indirect or direct objects

Question

i read from one of the question asked before in one of the grammar lesson, and it is confirmed that llamar is a transitive verb and takes direct object but comes with a preposition A because it is personal a. llamar + a + alguien.

But then i also came across llamar a la puerta for example. why is there a preposition A?

or did i get it wrong that it is not personal A but a fixed expression with llamar + a + any direct objects?

voy a llamar a Juan = voy a llamarlo, voy a llamar a la puerta = voy a llamarla.

But i did come across sentences that do not use preposition a for llamar.

le llamo idiota a juan - i called juan idiot. is this correct?

Other than calling people directly like telephoning or shouting for them, we can also call them names. is this where there wont be preposition A?

sorry for my errors as i am still not familiar with using the llamar verb.

Accepted Answer

Hola Ashlyn
Very interesting question...
You understand correctly the "personal a" with transitive verbs that use a direct object referring to people so "llamar a Juan" is correct and it means "to call Juan" (on the phone) 
But the verb llamar is also used in different ways. The ways you mentioned are correct:
-llamar a alguien algo/alguien
For exampleÑ

Me llaman la "reina de la discoteca" porque me encanta bailar. 
They call me "the disco queen" because I love dancing. 
A mi abuelo lo llamaban loco porque tenía ideas muy progresistas. 
My grandad was called (people called my grandad) crazy because he had very progressive ideas.

Llamar a la puerta is a set expression meaning "to knock on the door" - in this case "the door, la puerta" isn't a direct object. We use the preposition "a" to reflect the "on". 
Here is a link to the entry for "llamar" in the RAE diccionary. You will see all different ways to use this verb. If you look at number 11 you'll see they are referring to "knock on the door" as an intransitive verb. 
I hope it helps.
Saludos
Inma