The phone was ringing when I got home the other night, and a ringing phone was the last thing I needed. I had 45 minutes to get dinner for three children and then be off to a back-to-school night, and things were getting slightly frantic. I picked up the phone. It was my mother. "Calm down," she said.

Now my mother and I have an association that goes back a number of years, and she has an uncanny knack for knowing what is going on in my mind, but this was a little much. How had she known I was not calm? Moreover, what was wrong, under the circumstances, with not being calm?

"Mother," I said, "I just got home, I've got to feed the kids and leave in 45 minutes to get to back-to-school night. Why are you telling me to calm down?"

"Calm down," she said again. Then she told me that my son the 17-year-old had been involved in a car accident.

When you are paying insurance rates for a teen-age boy, the question you ask immediately after hearing he has been in an accident is a true test of motherhood. I asked how he was.

"He's all right."

Then I asked whose fault it was.

She didn't know yet.

Chief among my good fortunes is the fact that my parents live near me and have come to the aid of my family in all of our various emergencies. This time, unable to reach me, my son called his grandparents, and my father went to the scene of the accident. This, however, was cause for immediate alarm.

My father is the soul of kindness, and while this is a wonderful trait in a father, it is not necessarily a desirable trait in deciding who is at fault in an accident. I had visions of this being handled in a terribly civilized fashion, with a great deal of handshaking all around, and my insurance rates going through the roof.

Mother told me not to worry. So I proceeded to sit down at the kitchen table and worry until I got the next phone call. I began having a major crisis of confidence in my son the teen-ager, who had, up until that point, an impeccable driving record. I had seen indications, for example, that he was proceeding along the typical teen-age driving pattern: trading in caution for overconfidence. I had seen signs that he had inherited a genetic tendency toward tailgating. More and more often I had seen him leave our house without a seat belt on. By the time my mother called back with the next flash from the accident scene, I had tried and convicted my 17-year-old in absentia and confiscated his entire fortune in order to pay for the damages to the car and our new insurance rates.

I was, of course, guilty of the worst prejudgment. After a few more calls, it became clear that my son was not at fault. According to him, he had been driving along Rte. 7 on the way home from school at 6:15 in the evening when a woman pulled out of a gas station and into the right lane. Seeing her in his lane, he pulled over into the left lane. She then drove forward into the left lane, leaving him the choice of swerving into moving cars or her car. He braced himself, braked the car and crashed head-on into the left side of her car. The woman has been charged with failure to yield the right of way.

Teen-age drivers are involved in one of every five fatal accidents and one of every four accidents in which someone is injured, according to the National Highway Traffic Safety Administration. Some 650,000 of them are injured on the highway every year, but only 6.8 percent of them use seat belts. They are the most accident-prone and least safety-conscious group of drivers on the nation's highways. My son will now become part of the accident statistics but he will not become one of the injury statistics, which is the reason I am writing about this.

The car is badly damaged. My son, however, walked away from the accident with a bruised hand and a bad bump that he got when his head struck the roof of the car. On Wednesday morning, he went to the doctor, who is something of an expert on car safety and has lobbied for child restraint legislation. The doctor told my son that under ordinary circumstances, at the rate of speed he was going, he would have probably gone right through the windshield and onto the asphalt.

But the circumstances weren't ordinary, for which I will be eternally grateful. He is in school instead of the hospital because he was doing something most teen-agers, and most adults for that matter, won't do.

He was wearing a seat belt.