Apparently, if you’re browsing the web on your O2 mobile phone in the UK, the phone’s web browser sends your phone number to every site you visit using their mobile data network.
Here’s a simple script page you can visit on your smart phone to check whether your network is handing over privacte information too. The script prints out all the information that it sees when you visit. The site, of course, could be accruing phone numbers, but the author says he has no evil intent in mind and is simply revealing what your phone is revealing about you. Other sites are probably grabbing the same information with malicious intent (advertising!)
If your phone network is naughty you will see a line beginning with “x-up-calling-line-id” followed by your mobile phone number in plain text. If your number is not shown, you’re safe. If it is, it’s time to complain to your provider under the Data Protection Act.
Test your mobile phone for leaks: here.