One day I was trying to call with my mobile phone but again and agian my phone is disconnected.
So I just told this problem to one of my office friend.
After that he told me that it is network error, and then he asked to me that your Internet is working or not, so I just told him that yes it is work, after he suggested to me to install and downlond the Viber app;ication for free calling by using Internet after I just this, and seriously it works, Nice application for free calling.