With 'call', with the meaning you intend here, Ju, we use 'what'.
What do you call the kind of ...
With 'say/pronounce' we use 'how'.
"How do you say the kind of twins where ... "
Martha: How do you call your kids for lunch when they're all over the farm.
Jill: Oh, I use that big dinner bell on the porch. I got tired of yelling.
I believe that these types of twins are called Siamese Twins.
Student or Learner