Once you decide which types of disasters are most likely to occur, you need to determine which communication systems will likely fail, when, and how. This information allows you to create a disaster communication response plan that fits the need.
There are two general modes of communication system failure to consider. In a hurricane the first to occur is usually "call volume overload", followed by infrastructure damage and failure. A tornado will likely produce the same two results, but in the opposite order. This is significant for two reasons. In the hurricane example, you will have time to "ramp up" your support as the communication systems deteriorate. In the tornado example, the maximum effect will be almost immediate. For planning purposes, these two situations could be characterized as having "gradual" and "immediate" needs.
Telephone network overload occurs when the number of attempted calls exceeds the system's simultaneous call capacity. Most telephone networks are designed so that at normal peak loads, your probability of being able to make a call is 90% to 95%. When a disaster occurs, the call volume can increase more than a hundred-fold beyond the normal peak load, and the probability of being able to complete a telephone call drops tremendously.
Cellular telephone networks fail for the same reasons wireline networks do, with the additional liability of a more fragile infrastructure. Antennas and towers can fail, and microwave links between cells and switching centers can be damaged quite easily. Add in call volume overload, and you can quickly understand why cellular networks should never be relied upon as a replacement for landline (wired) networks in a disaster situation.
Public safety communication networks suffer from the same general modes of failure as telephone systems. In addition, public safety agencies are labor-intensive operations, and quickly run short of personnel to do the communicating. Available personnel work around the clock with adrenaline pumping, they become physically exhausted, tempers grow short, and the ability to cope with the disaster diminishes.
Organizations like the American Red Cross and Salvation Army have minimal communication systems of their own, and are not equipped to deal with the large volume of message traffic a disaster brings. American Red Cross has limited national communication assets that can be moved into a large disaster area as needed, but this takes considerable time and is seldom enough. Since they are generally dependant on telephone service and very limited radio systems in a disaster situation, these organizations are almost automatically in "overload" mode without outside radio communication support.