We know that the network of private investigators around the NOTW were able to locate mobile phones, by paying a corrupt copper £500 a dip.
If the copper could do that, they could also listen to the calls or, to be honest, do what they liked. Who was the copper? Was she or he the only one? Do we know how many requests came from them? What measures have been taken to secure the lawful intercept interfaces of UK mobile networks?