Twitter user @lewispeckover picked up the problem and created a simple website to show what’s happening. The site returns the information any website could poll from a user. The site clearly shows the the x-up-calling-line-id header is in full force when accessing the site via a mobile phone, but in actual O2 is sending out the information.
The company says it is “investigating this at the moment and will update everyone as soon as [it] can,” but customers on twitter have reacted with anger. Commenter’s pointed out the O2’s MVNO – GiffGaff and Tesco Mobile both get affected by the phone number sharing problem too.