Hi,
Grammatically, 'who' is used when you are denoting the subject/doer of the action. For eg. Who is shouting?, who is crying?, who is talking etc. 'Whom' is used in the nominative/ object form of the verb. For eg. whom are you talking with?, whom are you staring at?, whom are you calling tonight?, etc. The doer of the action in this case is 'YOU'.
Hence, (if I have understood the context correctly), the sentence 'Who are you calling?' is incorrect. It should be 'whom are you calling?'