The first meaning of the verb call is to use your voice to draw someone's attention. In relation to using the phone it can be used for the actual fact of dialling the number (and possibly not getting through...) as well as for the fact of having a conversation over the phone with someone whose number you have just dialled (you would not use it if you were on the receiving end).
In your example I personally think there are three possibilities.
If you want to refer to the actual fact of dialling the number then of course the choice has to refer to the past, by way of either using the past or the present perfect. I would use the present perfect because it is the tense that links a past action to the present, and the telephone call is still ongoing. But there might be local usages (US I think, but I'm not an expert on US English) that would tolerate the past.
I would use the present V-ing form because I consider that the act of calling covers the act of dialling the number + the act of talking to the recipient. Moreover in your particular example where you want the caller to come quickly to the point, using the present V-ing helps bring the caller back into the urgency of the action. So, I'd say :
Why are you calling?