Phone Hacking

With phone hacking taking over virtually all of the news programmes, I was idly wondering if the rest of the world has come to a standstill.
However my interest, as a retired electronics/computer engineer, is in the mechanism of phone hacking and how one can know whether one's phone has been hacked.
Firstly, this phone hacking has to be distinguished from phone tapping, which is where someone listens to an actual conversation as it takes place, and their seem to be no allegations that this has happened, mainly, I assume, due to the difficulty of intercepting the digital mobile phone signals.
In fact the phone hacking seems to be a matter of access to the victim's voice mail. With a mobile phone, as I understand the situation,  this is held as some form of digital recording by the phone company, and is accessed via a phone using one's password. I have never used this facility (and rarely use a mobile phone) but I note that messages on my "answerphone" at home can be accessed remotely if I should want. As I haven't set it up, and people leave us occasional messages, I assume anyone could access it remotely should they so wish. This would, of course, really be my own fault as I haven't set any password, but then who'd be interested to know that our daughter rang, and she'll ring again later.
What does interest me however, is how all these victims of hacking know that their voice mail has been accessed. There is no counter which might indicate the number of times a message has been accessed, and it would seem that the only way of knowing is in the event that someone has left you a confidential message which has become known to a third party. But this relies absolutely on the fact that the person leaving the message can be 100% certain that he/she has not mentioned it to anyone else, a rather dubious assumption in my view..
So How can anyone be certain that their Voice-Mail has been accessed?

Another strange aspect is that there has been no suggestions of hacking into e-mails.
Depending on the settings of one's e-mail client, these can be deleted from the ISP's server immediately they have been transferred to one's computer, although some users, myself included, tend to leave them there for a while. Either way, until they are deleted, they can usually be accessed as web-mail if the password is known. As the e-mail address of a potential victim is known, and complex passwords are rarely used (mine is limited to 10 characters), I would have thought it a reasonably simple task to work ones way through a list of potential passwords. A competent programmer could probably automate the whole affair.
And of course this gives rise to the same question as above,
How can anyone know if their E-mail has been accessed?

To me, answers to these questions would be far more interesting than the knowledge that various people believe that their phones have been hacked!