The fingers of the nation are poised at their telephones and the ears of the nation at their radios for "Ask President Carter," the big call-in show scheduled for 2 p.m. today on the CBS Radio Network.

But some people are going to be listening more critically than others.

There are those who do this sort of thing for a living, you know. They are the pop psychologists, loony baiters and the earnest counselors of call-in radio, and some of them are a little surprised to find the President on the chit-chat circuit with them.

"I think it's great that Jimmy Carter's getting in the same business I am," says veteran talk jockey Allan Prell of WEEL in Fairfax, Va. "But I'm not sure I like him taking the Presidency to this level, to this common a term. I always thought the Presidency should be if not imperial at least at little sophisticated."

Phone-call radio is not always a pretty world, Mr. President. There are some "cranks, lunatics and weirdos," to use Prell's phrase, just lying in wait. "Society is full of a lot of kooks," philosophizes Bob Grant, who communicates with many of them on WMCA talk radio in New York."There are some people who just shouldn't be on the air."

Some of such people could get past the elaborate technical screening devices set up for the broadcast and, despite a 7-second-delay device that allows for instant censoring, a certain amount of kookery could intrude into this broadcasting milestone. Then again, it's the kookery that usually makes the talk shows entertaining. Let's face it; talk radio is low art. It's even low radio.

What the professionally cantankerous Grant thinks is that Carter's version of Grant's act will be so tame as to be dull. "I should be doing the program with him instead of Walter Cronkite," says Grant. "If I felt that the President wasn't really answering a caller, I'd cross-examine him myself. Uncle Walter is just going to be very avuncular about the whole thing."

"I think it's going to be a minute of talk, a lot of mere formalities," says Prell, who was Mister Talk of Washington radio until his home base, WTOP, switched to an all-news format in 1969. Now Prell holds court in the suburbs.

True, the President won't be talking with the usual call-in radio audience, but Prell - who has experience in the matter - predicts there will be a large number of 'semi-inebriated' callers acting on dares.

Michael Jackson, the man at the mike and the telephone weekday morning on KABC in Los Angeles, thinks there will also be a lot of folks phoning in "just so they can say, 'Guess who I chatted with today.'" He describes hosting a talk show as "Russian roulette with the telephone."

"If it's going to be meaningful at all, President Carter should ask questions of his callers, not the other way around. That's the only way he'll learn anything," Jackson says. "He should remember that while he will be at east, they will not."

Jackson likes to give calls as well as receive them. He once dialed Idi Amin's number in Uganda and was put right through. He also called Golda Meir. He thinks the President should do the same. "He should call Idi Amin and put him on the air. I think it might be interesting to hear what he has to deal with when he talks to other world leaders."

Like many talk show hosts, Jackson is a living newspaper. He has to keep well informed so he can tangle with callers. He'd like to ask the President "what is the optimum he is hoping to achieve with the $50 tax rebate" and other toughies.

Carter aides are reportedly fearful that his off-the-cuff answers might set policy. But never mind that, Mr. President. The real question is, is this going to be a good show" Will it help put radio back on the map? Will it make call-in talk shows respectable?

"Most radio call-in shows," says Prell, "are 99 per cent entertainment and one per cent information. I expect this one will be the same."

Things can go wrong. When Jackson started in talk radio in San Francisco, "there were only two telephone lines, and the free speech people from Berkeley would tie them up so I didn't get any calls. I had to ad lib for six hours."

"I don't suffer fools and I hope Carter doesn't," says no-nonsense Grant. "Other people have failed at this job because they were too courteous. Carter shouldn't engage in small talk. He should keep the program moving. And above all, no electioneering. He shouldn't tell everybody how great he is."

Grant thinks the whole idea is a "gimmick," Jackson calls it "a noble experiment, like wearing sweaters and having fireside chats," and Prell, who ought to know it when he sees it, says of the Jimmy Carter phone-in. "I think it's great show business.

Rrrrrrringggg. Rrrrrrringgggg. Hello? Yes. Just a minute please.

It's for you, Mr. President.