President Carter stands to gain a lot a relatively little risk from his first "Ask President Carter" radio call-in show, scheduled for 2-4 p.m. Saturday over the CBS Radio Network.

If public reaction is favorable, Carter will have a new techique for taking his case, whatever it may be at the moment, directly to the people, a gambit his close friend and adviser Charles Kirbo thinks he will have to use in some of his projected battles with Congress.

Not even Franklin D. Roosevelt, the last President who mastered radio as a means of explaining his policies, could claim that his "fireside chats" were part of "a continuing effort to make himself available to the people of the country to answer their questions," as the Carter White House has done.

And no other President has had a mechanism to claim tht any citizen, no matter what his or her status, can pick up a telephone and have the same toll-free chances as any othe citizen of reaching the President.

The fact that the chance is incredibly small - perhaps millions of phone calls have to be funneled down to a single speaker in the Oval Office - may not detract from the symbolic openness of the event. The test will be whether the public likes the idea.

"Our concern is more breaking down the isolation of the President than anything else," said Barry Jagoda, Carter's television consultant." . . . The volume of calls that we receive is not as significant as the fact that millions of people will be able to hear those calls that actually do get into the Oval Office."

The risk to Carter's image is minimal because in six weeks as President he has shown himself very much at home with use of the electronic media.

He came across as relaxed, confident and determined during his fireside chat and his two televised news conferences.

Aside from some unforeseen presidential fumble with disastrous policy implications, the worst thing likely to happen is that the questions, which will not be screened in advance, turn out to be inane or repetitive or both.

Moderator Walter Cronkite, who will attempt to clarify confusing questions and keep callers from rambling, might be reduced to an endless series of pleas to "turn your radio down" if callers are confused by the standard seven-second delay!

The White House may have anticipated that. "Our assumption is that there may be those in this country who would be disrespectful to the President, but that no one would be disrespectful to Mr. Cronkite," said White House press secretary Jody Powell.

Powell didn't say what the White would do if millions of callers became angry or frustrated because they couldn't get through.

"If it doesn't work, we're not going to do it again," said Jagoda, a former cbs producer. "We're not afraid of failure."

The White House is making no claims that the callers will represent some average slice of America. Jagoda said Carter will be able to answer only 30 to 50 calls at best during the 120 minutes of air time.

Asked what the President might learn from this experiment, Jagoda talked about "the immediacy of the questions and the passion and the emotion involved in something that electronic communications somehow are able to convey better than seeing it on a piece of paper."

The mechanics of the call-in, while sophisticated, were not difficult for American Telephone & Telegraph Co., which says there is virtually no chance of the calls jamming phone equipment here.

Normal Saturday afternoon service should not affected anywhere in the country, an AT&T spokesman said, unless half the popilation of, say, Los Angeles, picks up telephones at the same time to dial the toll-free number.

Even then, the overload would remain in Los Angeles, causing problems for switchboarfs there, but would not be transferred to other parts of the country, the spokesman said.

The computerized system operated by Ma Bell acts like a series of giant funnels, picking up calls from 16,000 telephone exchanges and routing them through a series of switching points to 10 regional centers. At each stage, the switching points will give most of the callers busy signals and feed a limited number of calls to the next stage.

Chances for everyone are equal, the phone company says because each of the 10 regional centers handles approximately 10 per cent of the nation's telephones. Even Washington area callers will have to dial the special toll-free number, (900) 242-1611.

The regional centers will feed calls to 20 telephones in the Old Executives Office Building next to the White House, where CBS employees will ask for the caller's names and phone numbers, and check quiclky with information operators in those areas to make sure the callers give their own names.

Those who pass that check will be called back on a first-come, first served basis. Callers who dial from pay phones will not be called back.

Only one phone call at a time will be fed into the Oval Office,where Carter and Cronkite will be listening to a speaker. The President will be talking into a microphone so he won't have to hold a receiver to his ear for two hours. There will be no commercials.

The lucky few who do get through will be those who finished dialing just as a clear path to Washington opened up through all those switching and regional centers.

Everyone else will get either a busy signal or, in areas that have the equipment, a taped message saying all lines to "Ask President Carter" are busy.

CBS spokesmen say the network retains all editorial control and will pay all costs, which include and will pay all costs, which include $50,000 in long-distance line charges and some internal setup costs tht the network will not divulge.