If they were trying to blow up the phone building as the primary thing (instead of mostly trying to kill people), they might have just not made a big enough bomb, or they didn't realize how solid those old phone company buildings are.
My Dad worked for AT&T and its various predecessors and spin-off companies for decades, I even worked for him for a while. Those old Cold War era phone buildings are very, very solid, built to stay functional in the event of all sorts of emergencies.
In the late 1990's Dad was forced to retire. A few years later they rehired him and many other recent forced retirees as contractors for much more pay than they all made before being unwillingly retired - Ma Bell didn't anticipate network internet bandwidth needs and had to scramble to build up capacity.
That's when I got hired by Dad, when I was between jobs within my career field.
Those big buildings were mostly empty by then, miniaturization had allowed vastly greater capacity in a much smaller footprint. We were mostly installing huge banks of huge lead-acid batteries for backup power, plus thick copper cabling sewn onto racks. But jeez - those were some seriously reinforced buildings.
I would not be surprised if some 5G conspiracy person tried to blow up an old but still in use phone building and just didn't realize how big of a bomb one would really need to do that.
