Verizon was still out ten minutes ago. I think it is sheer volume of calls.
I read article where AT&T rep said it was due to the volume of calls, no damaged to equipment.
Modern communications can be really delicate to events like earthquakes, power issues, wars etc. In big events, cell phones/computers etc, are useless for most.
It's why I have HF ham radios, which all can be operated on 12 volt power. Eventually they will be solar powered to sustain/charge batteries etc.