loss of backup does NOT take down networks
VZ is disseminating.. if a fiber cut took out their 911 wireless capability, they had both primary and redundant communications in the same fiber duct. even for 911, the cost and complexity of engineering full redundant alternate path communications, in which nothing in the stream at all is ever within a shovel cut of any other part of the primary path, means it isn't done.
911 centers connect to 911 routing services, which are connected to phone companies, which you connect to when you grab the phone. to cut costs, increasingly, the routing services are contracted out to whomever. whomevers are not required to be as robust as phone companies.