Let's be straight here - there is no hacking going on exactly (ie. requiring special skills, social engineering, specially constructed tools or software), the only input is knowing someone's phone number and taking the chance that they've not set their voicemail PIN away from the default. Just phone the voicemail service for the network, enter the phone number and the PIN number.
BT have done way more in this arena than they like to let on about - look up "Phorm" - the UK government is currently being sued by the EU information commissioner for millions because they failed to adequately investigate BT's illegal interception of tens of thousands of it's own broadband customers without their consent (implied or explicit) for the purpose of BT making some cash by pushing targetted advertising at it's customers.
As a whole the practice of voicemail interception is not out - it's still no more legal than concealed camera/microphone stuff... all have a public interest defence. What is objectionable about this particular case is the broad scale fishing expeditions, interference with criminal investigations, collusion with the police (with or without inducements to do so) - I have no problem with covert camera or microphone or indeed illicit interception of voicemail messages of figures who put themselves into the public eye voluntarily (ie. not victims of crime) - in the genuine public interest (that doesn't include who's knobbing who in "celebrity" circles)
