It's an artificial intelligence, not an infallible god.

In the case of a base established specifically for research on dangerous software, connections to the outside world might reasonably be heavily monitored and low-bandwidth, to the point that escape through a land line would simply be infeasible.

If the base has a trespassers-will-be-shot policy (again, as a consequence of the research going on there), convincing the perimeter guards to open fire would be as simple as changing the passwords and resupply schedules.

The point of this speculation was to describe a scenario in which an AI became threatening, and thus raised people's awareness of artificial intelligence as a threat, but was dealt with quickly enough to not kill us all. Yes, for that to happen, the AI needs to make some mistakes. It could be considerably smarter than any single human and still fall short of perfect Bayesian reasoning.