word choice - How to ask for the name of something?

My question was retitled:

What is this function called?

Originally my title was:

How is this function called?

I still think that my version was correct; I always state such questions this way. Which one is really correct?

Answer

What do you call...? should be answered by a noun.

Q. What do you call your dog? A. Lucky

How do you call...? should be answered by an action.

Q. How do you call your dog? A. By whistling