1. Text messages can get through when voice calls will never get through. Even if you have zero bars, there's a good chance it will get through. Get some elevation if you can to increase your chances of reaching a non-congested cell.
2. If you happen to find a genuine payphone, international trunks may still be active, even if local and national ones aren't. Call a friend in another country, and ask them to relay messages to whoever you need. (I used this during 9/11 to bypass the damage in Manhattan. I called a friend in England, and he in turn was able to call my parents and let them know I was safe.)