Strange pop up with IE7

Hi, every now and again I get this pop up when I click the search box in IE7, It says "please enter your phone number" and only has an OK button. I have to open task manager to close iexplorer.exe rather than click it before I can do anything else.
http://imagegrotto.com/view-popupss8362.JPG
I'm not sure what this is, Computer is running well other than this. Seems to happen once every 2-3 days when I click my google search box.
Anyone seen this before?

Edit- Ok, having looked at the screenshot again, I think this has something to do with my homepage, it's my ISP and has the line checker there. I will contact them.