O2, a UK phone operator, seems to be sending along in the HTTP headers of their clients http requests the user's mobile phone number! Lewis Peckover has discovered this and set up a test page for people to see what information their mobile ISP is actually sending to websites.
To test, disable your mobile phone's Wifi and visit this page. There you should see the usual stuff, user-agent, IP, languages, etc.. If you see other things like your mobile phone number you might want to ask your ISP for explanations.
I am guessing this shouldn't work using HTTPS, however I would like to ask Lewis what is the deal on this as I do not have a mobile phone and hence cannot test this at all. I also don't have a Twitter account so... . If you do, ask him. Khtxbye : ]
Update: I found this old thread about the same sort of thing affecting other customers on other mobile networks. This is really not new and this is not an 02 issue but rather a mobile phone ISP issue.