Thanks NormanM. Your suggestion helped me. This is what hapnd with my premium subscription 12 months purchase. There was an option box for selecting Unlimited calls to USA and Canada. I remember I had chosen it before I enter into the payment page, but later some how, I accidentally unchecked the option box. When I rechecked my payment receipt, I realized that there was not Call to USA and Canada Item in the receipt. So I contacted to skype team by Online chat and they refunded my money back. I purchased skype premium again by choosing Calls to USA and Canada carefully. Now I can make calls. Thanks again.