Question

I know that someone is on the other end of the telephone line, but i have no real idea to __ i am speaking.

Answer 1

I know that someone is on the other end of the telephone line, but I have no real idea to whom I am speaking.

Answer 2

The correct answer is whom. "Who" becomes "whom" when it is the object of a preposition (in this case "to"). Since the antecedent is a person, the answer is not "what."