I have noticed weird popup ads appearing on pages which don't load those ads like daringfireball. The ad in question appears on the right side of the page over the web page intended to open.
The scary part about the popup is that it's correctly identified my fixed line phone number and asks for entering mobile number and email address as shown in the page. On inspecting the URL of the iframe, it points to this address : "http://www.adphonso.com/testmtnl.php?CSID=99999999@mtnl". Here, MTNL is the name of my ISP and I have inserted the "99999999" in place of my actual phone number which is there in the URL.
The problem I am having seems very similar to the one described in this question. The difference is the popup from that question looked like a scam ad and also it didn't display any personally identifiable information like it does in my case. I visited the site adphonso.com and it looks like an ad network, so I think it's mining for information. Should I be scared as the only cause suggested in that question seems to be malware or can it be my ISP is injecting code in sites I visit(which should be illegal if that's the case). But, I don't install untrusted softwares and every application I download, I usually scan them from virustotal.com before installing. Also, I have Avast Antivirus running on my PC. The hosts file seems to be fine and I use Google DNS servers. I would have tried using the fix provided in that answer but it doesn't seem to work with Windows 8.1.