The dependent system of the auto is severely tested during gas rationing days when we have to queue up to get fuel so the auto can move at all. The fuel delivery process is the dependent system that makes the automobile useful; i.e. so we can move about and inter-communicate with people and places.
The dependent aspect of the telephone is a network of switching centers and countless miles of wire that connects them. When these go down, the telephone - like the auto - is useless. It won't take us anywhere to communicate with anyone.
The dependent aspect of the cellular phone is a wave switching system that allows the movement from cell to cell, then to connect to a remote system to reach the desired number many miles away. When overwhelmed, as in emergencies when thousands try to use it, it can't do the job. It too, depends on the aforementioned landline telephone system. Most people forget that it is simply an extension of the POTS - Plain Old Telephone System. Due to the nature of the cellular system it is quickly subject to overload for several reasons, including the media types who call their office and leave the connection open to preserve the call for the duration of the event.
The dependent aspect of the computer system is the wires that make it work. When they are damaged (backhoe, train wreck, explosion along a telephone link, fire in the server room at the office) the system fails. The computer network is a complex system that can be brought to its knees quickly by the slightest glitch , a rampant virus, sheer ignorance or poor planning.
So, what does this have to do with emergency management? Just this: in the day-to-day use of these four systems it's far too easy to become so accustomed to them that we do not anticipate their non-use when needed the most: at the very time we need command and decision communications with TOP-LEVEL decision makers who essentially rely on one or more of these four seemingly transparent systems.