RENEE MONTAGNE, HOST:

Truckers see and hear a lot on the road, and there are two million Americans licensed to drive big rigs. So that's a lot of people in a position to notice when something at a rest stop doesn't look right. And that's why people who are fighting the sex trafficking of children are enlisting truck drivers to help. NPR's Carrie Johnson has the story.

CARRIE JOHNSON, BYLINE: Eight years ago, a truck driver parked at a travel center near Detroit made a phone call that changed a life.

WILLIS WOLFSWINKEL: And I pulled into a truck stop about midnight, getting my log book done, had two girls knock on my door, and I waved them on because I knew what they were looking for.

JOHNSON: Something about those girls bothered Willis Wolfswinkel. They looked young. So he called 911. And when the girls went inside another truck in the same lot, he called again. Wolfswinkel kept watching as the cops arrived.

WOLFSWINKEL: Evidently, this one phone call started a major investigation, because it turned out there were two girls that were, I don't know, 14 and 15, or something like that, that were kidnapped out of Toledo, and they, obviously, you know, in the end, were rescued.

JOHNSON: Wolfswinkel is 73 years old now. He's retired, living in a small town in Minnesota. Kendis Paris, who runs a nonprofit group called Truckers Against Trafficking, holds him up as an example.

KENDIS PARIS: So representative of how many truckers out there who are really wanting to do the right thing, ready to go and just needing to know who to talk to about this.

JOHNSON: Paris says her anti-trafficking group is reaching out to drivers to tell them about red flags - girls who look too young, for one, and then...

PARIS: Pimp control. You know, it's that SUV that pulls onto the lot, and three or four girls all get out at the same time and they're scantily clothed, and they begin to go from truck to truck to truck.

JOHNSON: Paris passes out cards everywhere she goes. On them is a phone number for the National Human Trafficking Resource Center, a 24-hour hotline.

UNIDENTIFIED WOMAN: National Human Trafficking Resource Center. How many I help you? OK. Could you tell me a little bit more about what makes you suspicious?

JOHNSON: The hotline is operated by a nonprofit group in Washington, D.C. Sarah Jakiel helped start the hotline about five years ago. She's watched it go from 200 calls a month to close to 2,000. Some of the best calls come from truckers because, Jakiel says, they're at the center of things.

SARAH JAKIEL: Incredibly unique position to recognize and report sex trafficking of children in this country. They're seeing it at truck stops, at travel plazas.

(SOUNDBITE OF TRUCK ENGINE)

JOHNSON: Here at this travel center near Baltimore, I chatted with some truckers as they fueled up. David Hathcox, who's been driving for five years, just dropped off a load of sugar. He says he's seen trafficking in New Mexico and California and Texas.

DAVID HATHCOX: They come knocking on the door of the trucks, or else they'll be hanging out up front or something like that. But as a rule, I try to stay out of truck stops like that. I have my fiancee with me, and, you know, because when you see that, there's a lot of violence and stuff like that.

JOHNSON: This travel center in Jessup, Maryland is huge. It's got a restaurant, a shower and this place, where I met William Heberling.

WILLIAM HEBERLING: And a laundromat. I just, I bring, pack enough clothes for four or five days, and I figure, you know what? I don't feel like doing my clothes when I get home.

JOHNSON: Heberling, a former Army Ranger, says this place is patrolled, nice and quiet. But he's seen some bad things on the road, too, like two girls in Kentucky who looked about 15 years old. Heberling called the police.

HEBERLING: You have to watch out for stuff like that. And usually, if you see somebody pretty young, it's something fishy.

JOHNSON: Every new employee at this TA Travel Center here in Maryland goes through training about human trafficking. Tom Liutkus, the director of nationwide marketing for the company, says it began doing a rollout last year.

TOM LIUTKUS: Humans are trafficked via virtually every form of transportation. And in some countries, it may be boats. In this country, it's more than likely to be by car.

JOHNSON: And that's a message his employees hear, mostly by watching part of this 30-minute video produced by the group Truckers Against Trafficking.

(SOUNDBITE OF VIDEO)

SHERRY: Thank God what saved me was that truck driver that called in and said, hey, you know, this is whoever at the TA's truck stop, you know, and we have some girls out here that look pretty young.

JOHNSON: That's Sherry. She's the girl who knocked on Willis Wolfswinkel's door that night eight years ago near Detroit. She and her cousin had been snatched by a pimp as they walked to Wendy's to get a Frosty treat. They were missing for months, until the police intervened after trucker Willy's phone calls. He told me Sherry's mother called to thank him.

WOLFSWINKEL: Oh, yeah. That was very touching. I know that, you know, I've got kids, and I can't even imagine what a parent would be or feel like if their kid disappeared like this.

JOHNSON: Wolfswinkel says if truck drivers say something when they see something that looks strange, maybe more parents wouldn't have to go through that pain. Carrie Johnson, NPR News, Washington. Transcript provided by NPR, Copyright NPR.