The technology of digital phones is supposed to make call interception more difficult. By chance I watched the Wire yesterday evening which had an episode about it taking more than a week to get a tap onto a 'pay as you go' mobile.
Today's Guardian allegations imply that there's other weak points to intercept.
My guess is that its through the voicemail, guarded by its massive 4 digit code, which for Orange used to be 1111, for Cellnet/O2 was 8705 and for T-Mobile was 1210. I guess if people change it the most likely substitution will be to their birthday date (ie DDMM* or MMYY).
So any malicious eavesdropper can take a punt that one of the default pins will work, or simply look up the person's birthday.
Three attempts should do it in most cases.
Press to hack rashbre central voicemail
* unless American-influenced, of course.