I dont know if you have read the article at BBC that I provided a link to in which ambassador denies the claim by Dawn (to which you provided the link) that it was hoax. Ambassador has said that call was identified and traced before it was transferred to Zardari and it was originated from Indian Foreign Ministry Office.
Now it may be some stupid high level officer of Indian Foreign Ministry and its duty of India to keep these unleashed hawks in control who are smelling blood and itching to go on war. I said high level officer because not every tom dick and henry in Indian Foreign Ministry office knows how to reach Zardari.