Hi! Pradeepto,
I would recommend that you seriously consider switching ISP. I have used sify in past and when you hit a problem its really difficult to get it resolved. Writing to list, making phone calls would prove futile, as the people managing the network are dumb heads (like most of the ISPs). Sify would have hired an equally dumbhead sysadmin or 'security consultant' who is blocking ports like anything. But problem with sify is, there executives dont listen at all. They cant understand things nor do they try to understand anything, unless you have a corporate connection. When things dont work out only resort with sify would be to move to consumer court (I know people in Delhi who have done this).
regards VK