Gamma rays also travel faster than the other destructive waveform. Much of the expansive destruction from a nuclear bomb is due to "over pressure", and these facilities have spring loaded blast valves that quickly snap shut when signaled via the gamma detector or a manual control to limit it from entering the protected zone.

Bunkers have different grading in terms of blast resistance and most of the AT&T bunkers were engineered for something like 5, 10, 15, or 20 mile air burst strikes of certain warhead yield.

The AT&T bunkers are mostly far removed from population centers such that a direct nuclear strike would be a waste of a perfectly good nuke. In reality, a directly targeted conventional warhead or sabotage would be plenty effective in causing major service disruption so I think a lot of the realities for survivability would be aftermath repair capabilities.