Any complaints could easily be dismissed as technical glitches. Yes the mobile phone service providers could easily log all the numbers that access the voicemail system in exactly the same way I log all the visitors to my web sites.
The first and most obvious method then if you wanted to access someone’s voicemail is to “borrow” the mobile phone, unless you was a close friend it would be hard to do this however if you left your mobile unattended maybe while you sleep or have a makeover it leaves it venerable to being used without your knowledge.
I guess that nobody ahs really considered that these messages are of any security concern.
Note home answering machines also suffer from this pin tone vulnerability and I wonder if the private investigators were also intercepting these.
Because this article ended up far longer than intended I will deal with tapping in another post however tapping usually refers to listening to or bugging of a phone conversation which is obviously very different.
Another point to note is the voicemail hacker will need to know what network the number is on.