Do you have an Internet-enabled phone? Are you with O2 (or one of the companies that piggyback their network, like Tesco, Giffgaff, etc)?
Then they’ve been giving out your phone number to every web site you visit while using mobile data.
O2 customer Lewis Peckover recently discovered that when you’re browsing over 3G on O2, your handset’s phone number is often included in the HTTP headers sent to each website you visit, in plain text.
Visit Lewis’s site to confirm that you are affected (at time of writing it was still active for me).
o2-oops
Note: this is not information that is added by your phone; it is added by O2’s web proxying servers as your web traffic passes from their network to the wider internet.