If war breaks out in the Middle East, some telecommunications experts worry that a sudden surge in phone calling by ordinary Americans trading the news could overload the nation's phone system and cause a temporary breakdown.

Many telephone companies are tightening security against sabotage by foreign agents as Tuesday's Persian Gulf deadline draws near. But many seem to view potential disruption by their own customers as a more plausible threat.

Phone companies are redoubling emergency preparations in hopes of minimizing the disruption of whatever calling frenzy occurs.

"A lot of folks are ... on edge," said Jim Nelson, network monitor at American Telephone & Telegraph Co., which carries 120 million long-distance calls a day, 70 percent of the U.S. total. "But there's not much more preparation that we can do. It's sort of what we do for a living."

News events often affect calling patterns, sometimes dramatically. When President John F. Kennedy was shot in 1963, telephone networks nationwide locked up as Americans tried en masse to dial friends and family.

The 1989 San Francisco earthquake set off a rash of calling to and from the California area. The busy signals that frequently resulted stemmed more from overload than quake damage.

What faces the nation now is potentially the biggest news flash since the Kennedy assassination in 1963. At AT&T's national network control center in New Jersey, technicians keep Cable News Networks on 24 hours a day, so that they will have a head start if war does break out.

"The AT&T network becomes, as certain events unfold, a technological extension of the calling public's psyche or emotions of the time," observed AT&T spokesman Herb Linnen. "The network signals -- sometimes within seconds -- the public's reaction in an era of instant news and communications."

Not everyone is convinced that gulf war would be the big one, however. US Sprint Communications Co. security specialist Jay Nelson predicts calling will not top Mother's Day, traditionally the heaviest day for long-distance calls. He believes news-related surges occur mainly because people call to check on the well-being of loved-ones. That won't generally be possible for the gulf.

Fred Briggs, senior vice president for network operations at MCI Communications Corp., predicted its long-distance system will not be overwhelmed. Larry Plum, spokesman for Bell Atlantic Corp., owner of the D.C. area's Chesapeake & Potomac Telephone Cos., predicted local calling would remain orderly. "We're at a heightened state of awareness" because of the gulf crisis, he said.

Today's networks are driven by computers, allowing technicians and complex software to monitor shifts in calling patterns and to take steps to prevent gridlock. If one trunk line is getting too crowded, some calls can be diverted.

When overloads occur, telephone companies play the role of gate-keeper, allowing only every second call, for example, to enter the network.

However, some analysts see weakness in the systems' size and flexibility. Their computer software, for instance, could in a crisis behave in unforeseen ways. Widespread chaos could occur, despite planning for it.

If one long-distance company's network gets tied up, consumers can try others. By dialing special access codes -- 10288 for AT&T, 10222 for MCI and 10333 for Sprint -- before their numbers, they can get onto networks that they do not normally use.

Network gridlock would be far more than an inconvenience for consumers, however. Most modern businesses depend in some form on instant communications. When a single AT&T cable was cut accidentally last week, for instance, financial markets in New York were forced to close temporarily.

In addition, the federal government relies on commercial long-distance for most of its telephone and data communications. In the event of a crisis, emergency plans call for government agencies to get priority for lines.