AUTOVON is an acronym for AUTOmatic VOice Network. It was a telephonic communications network operated by the United States government (specifically, the Department of Defense) from approximately 1963 to 1983.
In 1963, the DoD identified the need for a single, cross-service communications system distinct from the commercial telephone network (at the time, the one operated by the Bell System). The reasons for this are fairly simple. For one thing, the government didn't want to compete with civilian traffic for communications during emergencies, and it had already become apparent that in times of crisis humans instinctively reach out to each other - overloads of the telephone system during disasters and the like meant that just when the military would need that capability the most, it was least likely to be able to get it. Initially, as telecommunications technology became available, each of the U.S. armed forces branches had installed communications links haphazardly in a point to point fashion. This is (at least partially) because the first use of these systems had been on the battlefield, and the military was used to implementing communications as single point to point links. When permanent bases were set up and vastly expanded with the buildup for World War II, the various bases and services implemented links to their nearest neighbors both geographically and in the chain of command, and those circuits were managed by human operators much as the civilian telephone system handled it.
During the Cold War, however, the military began to realize that its preferred methods for strategic communications - namely, human-mediated telephone circuits and telegraph/teletype over point to point links - were just insufficient for handling a modern conflict. Nuclear weapons meant that the radio spectrum would be extremely unreliable, and the civilian telecommunications infrastructure was very, very vulnerable, relying as it did on fragile point to point microwave towers and cable switching centers. Those, for efficiency, were usually also centrally located in high population areas (a.k.a. primary targets).
The U.S. Army had a small switched telephone system at the time, named the SCAN system (Switched Communications Automatic Network - the military loves them some acronyms). It was a small (I believe three or four-switch) setup which managed connections between several of their major bases. The DoD selected it as the core of the new all-service system and in 1963 renamed it AUTOVON. From that point forward, it was the specified communications network for U.S. military installations.
The AUTOVON system operated 'alongside' the civilian telephone system. It differed from it in a few significant ways. The first was infrastructural - whereas the Bell System network (and, eventually, competing networks like MCI and Sprint) used cable runs between a relatively small number of large switching centers located in populated areas, manned and easily accessible for servicing, AUTOVON's interconnect points were small, with only a few circuits in and out, and were generally located in the middle of nowhere. In addition, they were typically buried underground, in hardened shelters. So, for example, three or four AUTOVON circuits might terminate in an unmanned underground concrete vault somewhere in a field 250 miles outside of Sioux City, IA. This increased the survivability of the network in case of large-scale attack.
Also, the AUTOVON system had a vastly larger number of cross-connects. Where the civilian phone system was basically a hub and spoke system designed for efficiency, AUTOVON's network was much more of a matrix, designed for resiliency. The AUTOVON switches had many more routing options than the civilian network, again specifically designed to deal with damage to the network from incoming attacks. Although running on specific dedicated cable runs, many of these individual runs were contracted from and maintained by Bell and other civilian companies, with those companies allowed to use a specified subset of the run's capability during normal operation.
AUTOVON used exclusively crossbar switch units. These units, at the time, were generally too expensive to use in smaller communities (such as those the size of a military base) but hey, defense contracts! The first standard AUTOVON switch was a version of the 5XB ("Number Five Crossbar Switch") designed by Bell Labs in 1947.
AUTOVON was not limited to the CONUS. It ran all over the world, eventually - wherever the U.S. military went and was officially known as "490-L Overseas AUTOVON Switching System." The first overseas switching center was installed in, I believe, Hillingdon, UK and was brought online in 1969.
As you might imagine for an organization as bureaucratic as the U.S. DoD, AUTOVON services were cross-billed in order to manage budgets. By all accounts I've run across, the billing system was even more insanely arcane and complex than regular phone bills, reflecting as it did silent but vicious attempts by each service to avoid paying for things.
On the consumer equipment side, AUTOVON had one significant difference from the regular telephone system. In addition to having a standard telephone keypad, AUTOVON calls could be made in one of five modes. These modes indicated the priority of the signal, and were (from least to most important): Routine, Priority, Immediate, Flash, and Flash Override. These modes were indicated by prefixing the number dialed with an additional DTMF tone, using one of four buttons on the keypad, labelled P, I, F, FO (if available). Restricting the priority available to a station was done either by restricting the tone buttons on the handset, or by filtering at the switch level.
These modes were part of what was termed MLPP, for Multi-Level Precedence and Pre-emption. It implemented a ranking system for circuit availability. If a call came into a switch and there were no circuits available between that switch and the destination due to them all being busy, the system would automatically drop any available calls in progress with a lower priority ranking than the incoming call. Flash Override was reserved for emergency communications from the National Command Authority.
It should be noted that the AUTOVON system did not provide any form of communications security other than that inherent in using separate, physically-monitored and maintained circuits. There was no encryption or other protection at the system level. Eventually, scrambling systems would be implemented in the handsets themselves, using modem protocols or other systems - since AUTOVON was an audio-only network, it didn't care what was being passed over the line so long as it didn't trespass into DTMF frequencies.
Using AUTOVON was relatively simple for the end user. Generally, in military installations, it was implemented using a separate prefix digit. In most bases, dialing '9' would connect your call to the outside local telephone network. Dialing '1' would connect it to the Bell System long-distance network, just as a civilian phone would. In both of those cases, using regular civilian telephone number systems would work. However, dialing '8' (or, in some cases apparently '88') would connect your handset to AUTOVON, at which point you would enter an AUTOVON-specific telephone number to reach your desired party. Exchanges in AUTOVON were scoped to military installations and units, and 'area codes' to military networks.
In 1983, AUTOVON was deprecated in favor of the newer Defense Switched Network or DSN, implemented using digital technology and computerized switching. Concepts from AUTOVON survive today, though - the Precedence field of the TOS octet, part of the IPv4 header, was a three-bit field which implemented six different levels of precedence for the associated network packet. Their names? Routine, Priority, Immediate, Flash, Flash Override and Critical. This survives into the more modern DiffServ QoS scheme described in RFC 2745, where the first three bits of the DiffServ Class Selector Code Point (DSCP for some reason) is, you guessed it, identical to the IP Precedence field.