UK network O2 appears to be submitting your telephone number to any website you visit whilst using the data connection on your phone. According to report from TNW, the information is transmitted when a user asks for a webpage view.

The issue was brought to our attention to Lewis Peckover, who created a simple webpage to check the information that a mobile browser would send to a website when it requested data. Whilst most of the data was to be expected, including the Host, User Agent, Referrer and Encoding, there was also another field in the results — x-up-calling-line-id. What is x-up-calling-line-id? Your mobile phone number.

O2 hasn’t acknowledged the issue yet but does admit that it is looking into the problem. Its Twitter account is currently getting deluged with unhappy customers seeking an explanation. Everyone asking about the problem is currently receiving the same stock answer.

We're investigating this with our internal teams, and will come back with more as soon as we can

O2 was the first network in the UK to offer the iPhone and had exclusive rights to it for the first few years. As such, many iPhone users are still using the network.  O2 and GiffGaff which runs on the O2 network are the only carriers that submit this type of information, according to TNW's tests.

Source: TNW