First the obligatory rant about how an ignorant media changed the meaning of the words hack and hacker from something to be admired to something quite different. Still rankles that.
That said, whilst I may misunderstand, from what I've read my take on this is that the phones themselves do not get compromised; the unauthorised access would be to the voice mail messages folk leave on the service provider's equipment. As I understand it, you call a mobile, then when the line is busy, call it again to get forwarded to the voice mail. At which point I can only assume that the security on some voice mail can't be what it should be, as it appears the caller can access some admin type functions if they know what they are doing, and listen to the messages. One would hope they have sorted any such loophole by now.
But I'm willing to be enlightened if that is incorrect.