[DanielDent]

At one point, one of the attack vectors was said to be by spoofing or otherwise manipulating traffic to/from a wireless femtocell. The same wireless femtocells that many operators will gladly sell or give to you for free or very little $.

I don't know how effectively that strategy still works today, but I think it speaks to the significant degree to which the phone system is completely insecure and untrustworthy by design.