During the 1987 quake of SF, I was in the Bay Area. For some strange reason, folks in the southern part of CA could NOT contact by phone anyone in the SF Bay Area. They'd dial their friends or loved one's number, and they'd get... me. But I could get their loved ones -- I could contact them. So, I operated a "communications relay system" between Bay Area and So California for almost 24 hours -- until the phone lines were up again.
Never figured it out, was amply glad to oblige.
The phone companies implement traffic controls to ensure that calls for help from inside the disaster zone get priority. Calls into the disaster zone are considered to be less important.