If you're planning on calling a radio talk show, be prepared. Depending on the type of show, you'll have to be ready for a long wait, unexpected jitters, a verbal challenge from the host or getting cut off.

The easiest show to get a call through: local shows on an unfamiliar topic. One Carnegie Endowment author who recently made the circuit speaking about the spread of nuclear weapons to small countries, said long periods would pass with no calls at all. "People don't have a firm opinion as to what needs to get done and who's screwing up the works."

In more popular areas, such as money, sex and personal problems, the odds of getting through are low. A good psychic can bring down the house. Reaction, however, can be hard to predict.

Says Kojo Nnamdi, host of WHUR's "Newsmaker" (FM, 96.3), "We always think we'll get a large response to drug abuse and we don't. If we do something on the budget, which we think is so detailed nobody will call, we get a large response. We can depend on a big response to male-female relationships."

Nnamdi admits he can be "pleasantly surprised" about the number of callers, "because there are some days when you say, 'I don't have that much to say, I hope somebody calls.' "

On the Larry King show (locally, WTOP-AM 1500), an estimated 1,000 calls are attempted each night, and about 200 get through on their 13 lines. Ringing is a sign you'll get through.

Norma Romeo, program coordinator for NBC's Bruce Williams show (in the Washington area, WWRC-AM 980), answers 400-500 calls a night on their six nationwide lines; 30 to 35 finally get on the air.

Callers to Talknet have been getting a busy signal or ringing, estimates Romeo, "anywhere from five minutes to three hours. These people -- their anxiety level has heightened to a point where they have finally gotten a human voice on the other end and it's desperation; please don't hang up on me.

"When I do accept a call -- put them on hold, find out where they're calling from, how old they are, all the information I need -- I don't keep them holding for more than 15-20 minutes."

What's the criteria for call selection?

"Interesting, unique and a geographic mix," says executive producer Maurice Tunick of NBC's Talknet. "There is something in your gut that tells you this is a good call or not -- the sound of their voice, the urgency in their question.

"On Bruce Williams, the urgency may be that they're in a terrible dilemma -- they just put $10,000 on a crooked deal. On Sally's Raphael, late-night Talknet therapist show, maybe someone just said yes to getting married and they're having second thoughts."

Says Romeo, who has a "dual audio system" with one ear on the caller and one ear and both eyes on Williams: "You try not to give people the opportunity to argue with you. It's very much like a negotiating stance."

"Just urgency alone is not enough to get you on the air," says Tunick. "Be persistent. It's extremely hard to get through."

Once you've made the connection, speaking up may be a problem. "Kaleidoscope" talk-show host Diane Rehm (WAMU-FM 88.5) says that as a caller, "I used to be far more nervous on the other end of the line than I am here in the studio. I don't know what that's all about."

When Rehm opened the phones to find out why more men called, many callers agreed that men have more experience in articulating themselves.

Wrote one Alexandria woman, "I was raised to let my older brother do the talking. It took awhile for me to realize I had strong opinions I wanted to express."

For people "who are nervous or who are apt to wander about," Rehm suggests "writing a few key words down. The callers that I appreciate most are those who have at least thought out what it is they want to say."

However well prepared callers are, their chances of getting cut off -- if they ramble too much -- are high.

"I have come to think that this is less and less a business of interviewing and more and more a business of interruption. I try to do it nicely," says Nnamdi, but he "constantly" cuts callers short.

"When I become a little impatient," says Rehm, "it's because a caller will have long pauses to think about what he or she is going to say and that's a waste of air time.

"Pace of the show is so important. I really do feel that courtesy is imperative, but at the same time I have an obligation not just to the listener on the telephone, but to all those thousands of other people who are listening."

For those determined to get on a show there is a high-tech recourse.

"Some people have one of those memory dials on their phone," says Romeo, "which makes it frustrating on my end because a caller I will sort of 'burn off' can get through to me over and over."