Walking along the Strip here, a visitor passes numerous open-air pay telephones where he can eavesdrop on the conversations. Remarkably, they all sound the same.

"Two, 10, 6 1/2," a caller will be saying, "Four, 3 1/2, seven, 10 . . ."

What the New York Stock Exchange ticker is to American finance, those pay phones outside the big bookmaking establishments are to the American gambling business. This city is the communications center for an industry of incalculable size. "It's unbelievable, said Sonny Reizner, who runs the sports book at the Castaways Hotel. "It's become so big that it's beyond my wildest dreams."

The sports books here are the hub of the industry. Point spreads on football games are set by prominent local oddsmakers like Reizner and Bob Martin of the Union Plaza Hotel, who are constantly monitoring not only news that might affect games but also the flow of money around this town. "I may move the line just because respect a particular customer," Reizner said. m"One man might move the line with a $2,000 bet and another man won't with a $5,000 bet. You've got to pay attention to everything. In this job, you can't blink. If you blink, you're in trouble."

The point spreads at the major sports books are steadily watched by the mysterious men at the pay phones, who report fluctuations to the nation's rapidly growing number of sports services. The services sell this information to their customers (presumably out-of-town bookmakers) and also sell their handicapping opinions to bettors.

It may seem surprising that in a game where even casual fans find it easy to generate their own strong opinions, so many people are willing to pay for somebody else's football selections. But the multiplication of the sports services has been phenomenal, and they have had a profound effect on America's betting habits. Last week, their impact was especially dramatic.

Mike Warren, a cynical Baltimore based tout with a large clientele, released Stanford, a 13-point favorite over California, as his service's "Lock of the Year." By the middle of the week, Stanford money started hitting Las Vegas like an avalanche.

"We took many $5,000 bets on the game," Reizner said. "We opened it at 13 1/2 and when the price moved to 18 we took it off the board (Elsewhere around town, the point spread went as high as 22). The thing I resent about sports services like this is that they get people so heated up that they want to mortgage their cars to bet. People were coming up and asking, 'Can I really bet on that game?' Like we were giving money away."

They weren't giving it away last week. Warren's "lock" not only failed to cover the big point spread, but lost the game outright.

The excesses of many tipsters are deplored by the reputable people in the business, one of whom is Jim Feist, proprietor of National Sports Services. "Ninety-eight percent of the people in this business are incompetent," Fiest said. "I do not like my competitors."

Feist started handicapping sports when he was growing up in Philadelphia and doping-out basketball games as a hobby. After he had started betting, he subscribed to a number of sports services, found their performances to be uniformly unimpressive, and finally went into business for himself in 1975. Even competitors acknowledge that Feist's operation is honest and his handicapping very capable.

He operates out of an office here that is piled high with newspapers, magazines and statistics, but he said he minimizes the importance of cold figures. "I try to evaluate everything from the emotional standpoint," he said. "All the statistics in the world will you not make you a winner. Everything is psychological."

Feist also monitors closely the movement of point spreads on games, for he knows that this is a town where smart money emphatically does exist. His phone rings frequently with calls from the Strip phone booths. "I'm constantly atturned to changes in the line," he said. "If I like a game strongly and the money moves against me, it tends to make me question my decision again. If I like the side they're moving on, and I can find out why it's moving, I'll push it or play it stronger."

In Sunday's NFL lineup, Feist has four opinions that provide some insight into the way he thinks and handicaps.

New York Jets (+10) at Las Angeles: "Even though they've had a disappointing season and aren't going anywhere, the Jets are young and enthusiastic and they're playing out the season on an upkick. I think their defensive secondary will be punished by (Vince) Ferragamo's passing, but the Rams haven't been scoring a lot and in the NFL today you don't lay 10 points against a cometitive club."

San Francisco (+7) vs. New England: "This is an injury game. I don't think the Patriots are going to risk further injury to their quarterback to roll up the score. You're not going to see that many passes."

Cleveland (+3) at Houston: "The Oilers beat them when they played on Monday night early in the season, but the Brown's offensive was out of synch then. Now, Cleveland is on an upkick. They're going to put points on the board. Houston is going to be forced to score a lot and may not be able just to grind it out with (Earl) Campbell's running. Plus, you have the revenge factor coming from the TV loss."

New York Giants (+2 1/2) vs. St. Louis: "My only question is why the Cardinals are favored on the road. They've done everything possible to lose games this year but they're favored because the Giants beat them in St. Louis and everybody expects them to turn the tables this time. I'd have made the Giants a two-point favorite."

"I should go four for four on those games," Feist said, then quickly laughed at his own hyperbole. Like anything on which men gamble, football has no infallible handicappers, no "lock" betting situations. It is a game of percentages and narrow edges, where mere points and half points are crucial. oThe boys at the phone booths understand that; so do the oddsmakers like Reizner and his high-rolling clientele; so do sports-service operators like Feist. The people who don't understand are the gullible customers of the disreputable sports services, looking for certainties and easy answers in a game where they do not exist.