When Lorraine Brooks made her daily morning phone call to Virginia Mixon two weeks ago, Brooks knew that something was wrong.

"Her speech was not clear. She was incoherent," said Brooks, who had become familiar with Mixon's voice as they became telephone friends during the past nine months. Brooks immediately called the manager of Mixon's apartment building and asked him to check on her friend.

He found Mixon conscious but becoming more incoherent. He realized she was having a stroke. She was rushed by ambulance to the Washington Hospital Center.

Brooks and Mixon have never met but they are part of a growing number of persons who have become acquainted through the increasing number of volunteer groups that call elderly shut-ins to make sure they are safe and well.

If a phone buddy gets no answer or believes that something is amiss, the volunteer calls someone who can immediately check on the senior citizen.

While no one is sure how many telephone buddy systems exist in the city, groups ranging from churches to the Red Cross, hospitals and senior centers sponsor them free of charge.

"There are so many elderly out there in our society who don't have relatives," said Joyce Palitano, who runs the 2 1/2-year-old Telecare Program at Providence Hospital. "They are very isolated and very lonely," and one phone call could make a lot of difference to people who do not talk to anyone else all day.

"They [the telephone calling services] make for a sense of security in the elderly to know that someone is going to call you and ask, 'Are you all right? Did you take your medication?' " said E. Veronica Pace, executive director of the D.C. Office on Aging.

The District, according to the 1980 census, has 102,881 residents 60 years of age or older, down from 103,945 in 1970. One-third of the elderly live alone, and the vast majority of them are women, said Sharon Hennery of the Office on Aging.

Most shut-ins and their buddies talk for 15 to 20 minutes each day.

Brooks, like many of the volunteers, is elderly. She worked as a social services representative with the city Department of Human Services until her retirement in 1978, she said.

"I am limited in my activities now, so I had been looking for some volunteer work that I could do from my home," she said during an interview at her Upper Northwest home. "I read about this Telecare Program in the paper."

Palitano, who runs the Telecare Program, said, "There are about 120 people who are called every day."

Many of the first recipients of the calls were former Providence Hospital patients. "Now some senior centers pass names on to me, but the majority of referrals are still through the hospital," she said.

Myrtle Yearwood, the counseling service coordinator at the Barney Senior Center on Columbia Road NW, runs a program there called Telephone Reassurance.

Nine months ago, one of the volunteers could not reach a man who was known to be a heavy drinker. The telephone buddy asked someone in his building to check on the man.

"He was found lying on the floor. He had passed out and had had a stroke," Yearwood recalled. He was rushed to the hospital, she said.

"We were told he would not have survived if he hadn't gotten help quickly. After that he had to be placed in a home," she said.

Juanita Thornton, at 71 a leading advocate for the elderly and chairman of the Mayor's Commission on the Aging for six years until her retirement last year, said many elderly residents need the buddy programs but do not participate because they cannot afford telephone service.

"Many of these people can't pay their phone bills so they have lost their lifeline," said Thornton. She cited rate increases granted the telephone company, and she said she spends a great deal of time lobbying against such increases.

"When you talk about a hot line, that's what the telephone is, particularly for the frail, the vulnerable and the homebound," Pace said.

Any District resident 60 or older who is interested in finding out more about these telephone service programs can call the D.C. Office on Aging public information service at 724-5626.