I used STU-IIIs for much of my career, and the technology steadily improved. In the early days of the system (late 80s), the units were large and it sometimes took two or three attempts to get the phones synchronized, encrypted and secure. On other occasions, there would be distortion on the line, suggesting the synch wasn’t quite right, so you’d hang up and try again.
But the technology improved steadily and the STUs in service when I left the spook world (2008) were vastly improved. Just place the call, push the button to “go secure,” then watch the display to confirm that the connection had been established and the classification level the network could support.
It would take a fair degree of technical incompetence/stupidity to be unable to operate a STU-III. But then again, Hillary is anything but tech-savvy, as evidenced by her inability to operate a fax machine and how long have they been around? Not exactly cutting-edge technology.
Then again, sending faxes, setting up secure calls and similar chores are tasks for the little people. Why should anyone expect Queen HRC to know how to operate a STU-III or secure Blackberry?
I left several years before you did (and went to mostly political polling, rather than other uses of math), and the STU was awkward back then. If Hillary was unwilling to switch from a Blackberry to an authorized but high-performing phone, that tells us something about just how indifferent she was to the safety of the SEALs, CIA staff, and others who died on her watch.