Many fallout shelters and other hardened facilities, including AT&T bunkers, used a system that automatically sealed up the facility upon detection of a nuclear detonation.
From what we've been able to piece together, here is how they might have worked at the AT&T sites.
Over pressure blast detectors were used to trigger the closing mechanisms on the various access doors and air intakes on hardened structures. This was called the Inlet Blast Valve system. Blast valve units are installed in each opening of the facility and close under the positive blast over pressures listed in the unit specs. After closing they can be manually reset at the blast detection control panel. Each valve has its own detector although they were tied together in a system known as "sync-closing," one detection would trigger all valves to go closed.
The over pressure blast detectors work by measuring the static ambient pressure from the protected zone with zero wind or weather anomalies. This it the set point and a blast over pressure registered between the static sensor and the Valve mechanism activates or releases the spring tension and slams closed the air inlet blast valve. Most sites included Gamma detectors that were designed to detect the radiation wave as well. They were redundant systems, any detection, overpressure or Gamma would button-up the site at which point signals were sent to all Continental U.S. sites that a blast was detected, where it was, the size of the blast and wind speed and direction. Sites within 250 miles of any detection would go to Auto-Lock down.
Upon the sensing of the pressure wave all the valves would seal their openings to prevent entry of undesirable environmental conditions. The trigger could be natural as a large lightening strike or man made as a nuke blast. Many false alarms were recorded as the result of lightening strikes which gave the operators quite a concern.
All underground hardened AT&T sites were designed to withstand a blast 2 miles away but were not designed to take a Ground Zero hit. At first 50 psi over pressure was the spec but then some were de-rated to 30 and others down to 20 overpressure. Keep in mind an over pressure of + .5 psi could destroy a wood-frame house in 1 second. Radiation monitors were place in service during any activations of the Blast detection System.