The phone call left veteran investigator Phil Merrill “shocked.” But also, he admits, just a tiny bit exhilarated.

A tipster had phoned the 24-hour hotline to report a sighting of the villain Merrill has spent a career hunting.

The perp was hiding in a paper recycling plant in Calgary, Alberta. When investigators arrived, they made a surprising discovery: He wasn’t alone. Authorities eventually rounded up all six of the gang.

It wasn’t easy, Merrill says: “It’s hard to see a rat with all of the paper.”

Pest-control authorities from Washington, D.C., to Wellington, New Zealand, are working to battle growing rat populations. They might want to meet Merrill.

From his office in Lethbridge, the loquacious 68-year-old leads Alberta’s much lauded, but never replicated, rat-control program. Key elements: zero tolerance, free poison and the Rat Patrol, a team of officers armed with shotguns, defending the Alberta-Saskatchewan border from the small invaders.

Next year, the province celebrates 70 years rat-free.

Now, a caveat: “When we say we’re rat-free,” Merrill says, “it means that we don’t have a resident or breeding rat population.”

Rattus norvegicus — the brown rat — is a destructive force in much of the world, wreaking ecological havoc, contaminating crops, vandalizing property and spreading as many as 35 diseases.

The species arrived in North America aboard the sailing ships of the late 18th century, but Alberta did not worry much about the pest until the summer of 1950. That is when a colony was discovered on its doorstep in the border town of Alsask, Saskatchewan. A hit squad was dispatched, and the infestation was eradicated. But the rats returned.

“For the first time in the history of Alberta, rats have infiltrated the eastern border of the province to the point of establishing colonies in hamlets and farms,” David Ure, the provincial agriculture minister, warned in a 1951 report.

Officials decided they needed to act quickly and ruthlessly to protect their citizens and their crops from the little miscreants.

Alberta is protected by bitterly cold forests to the north, mostly barren land to the south and mountains to the west. The government focused its efforts on the Saskatchewan border to the east, setting up a Rat Control Zone, where armed investigators kept watch for any rat that dared put its paws on Alberta territory — and snuffed it out.

From 1952 to 1953, nearly 70 tons of tracking powder was blown underneath the permanent buildings in the Rat Control Zone. Civilians were required to rat-proof their properties and kill any rats they saw. Authorities provided the poison warfarin free of charge.

But there was a problem: Most Albertans did not have the faintest clue what their enemy looked like. So officials paraded rat specimens around the province, distributed pamphlets on how to exterminate them, and displayed anti-rat posters at post offices, grain elevators and schools.

“You can’t ignore the rat,” one example reads. “He’s a menace to HEALTH . . . HOME . . . INDUSTRY . . . KILL HIM!”

Lianne McTavish, an art history professor at the University of Alberta, co-authored an analysis of the posters in 2011. She found that they were modeled after World War II posters that rallied the home front in the war against a common enemy.

In a 1954 pamphlet, O.S. Longman, the deputy agriculture minister, admonished Albertans who were “providing the rat with comfortable shelter and plenty of food.”

Within a decade, the number of infestations began to drop significantly, and Alberta declared victory. But the province is no less attentive, its defenses no less alert. Rats sometimes hitch rides on planes, trains and automobiles, or sneak in on foot.

Another caveat: Zoos and researchers may keep rats. But they need a special permit from the provincial government.

Pet rats? They are illegal. A family new to Calgary learned that lesson the hard way in 2010, when it was forced to surrender Matilda to authorities. She was spared a death sentence when neighboring British Columbia offered her sanctuary.

Merrill’s team still patrols the Rat Control Zone. They conduct twice-yearly inspections of farms and buildings along an 18-mile wide stretch of the Alberta-Saskatchewan border, looking for the telltale signs: droppings, chew marks, tracks.

“Some of the guys like to [shoot the rats] because it’s fun,” Merrill says. “It’s probably easier to just bait them out.”

The government’s 24-hour rat hotline (310-RATS) logs hundreds of false alarms each year. Accordingly, it dedicates a full page on its website to “Animals Mistaken for Rats” — pocket gophers, red squirrels and muskrats, to name a few.

The discovery of a single rat does not trouble Merrill’s team much because it can be trapped or baited fairly easily. But infestations — two or more rats together — are cause for real concern. There are a handful each year, and they often garner news coverage here.

Merrill has counted three infestations this year: two along the border that were not dealt with effectively last year, and the one in the Calgary paper recycling plant — a rarity in a major city.

“We get a little excited because we don’t get too many,” he says. “I won’t say it’s fun. But everyone’s on high alert.”

Saskatchewan has its own ­rat-control program. Richard Wilkins, a specialist with that province’s Ministry of Agriculture, says Alberta does an “excellent” job of controlling rats. But he thinks it’s taking some liberties with its definition of rat-free.

“A rat’s a rat,” he said. “If they want to blame us for the rat population, we could blame Manitoba, Manitoba could blame Ontario, Ontario could blame Quebec.

“They have them on the West Coast. They have them on the East Coast. They’re intelligent and adaptive. They can make a home anywhere.”

There are those who speak for the rats.

Trev Miller, an organizer with the Calgary Animal Rights Effort, says it’s “very important” that Alberta remain free of pests. But the rat-control program, he says, is too punishing.

“It’s a very difficult situation,” he says. “The current program is very cruel.”

Lisa Hutcheon, president of the Small Animal Rescue Society of British Columbia, says Albertans and soon-to-be-Albertans have contacted her organization a handful of times in the past decade looking for refuge for their pet rats. Some worry that even if they do manage to successfully hide their pet rats from the authorities, they will not be able to get them medical care if they fall ill.

Hutcheon says rats are her charity’s most popular pet for adoption. She says Alberta should draw a distinction between wild rats and domesticated rats.

“I really don’t understand it,” she says. “They’re really affectionate, they love their people and they’re amazing animals. I don’t think a lot of people appreciate them enough.”

In 2012, an infestation at a landfill near Medicine Hat, Alberta, dominated several news cycles. Officials set up infrared cameras and bait and unleashed a pair of bull snakes. When that proved futile, they launched “Operation Haystack,” in which they deployed bales ofhay laced with poison across the city.

Another caveat: Alberta is not the only jurisdiction in the world that has effectively eradicated rats. South Georgia Island, a British overseas territory, and Macquarie Island, a reserve of the Australian state of Tasmania, have dumped tons of poison in the last decade to snuff out the vermin.

But there’s something different about Alberta.

When McTavish moved to the province, she says, she learned “pretty quickly” about the rats.

“It’s part of the provincial identity,” she says. “It’s part of the way people understand Alberta.”

The France-size province of 4.3 million is said to be the largest jurisdiction in the world to rid itself of the rodent.

Merrill believes the vigilance is worth the effort. The program costs less than $377,000 per year to administer. In 2004, the ­government-funded Alberta Research Council estimated that it saves more than $31 million in economic and environmental damage.

Alberta benefits from geography. Banishing rats is much more difficult for port cities with large coastlines, which nurture much of what rats like to eat. But Merrill believes it is not impossible to replicate the province’s success.

He has been intrigued by the Ekomille, a contraption being rolled out in Brooklyn that attracts rats with bait and then drops them through a trap door to drown in an alcoholic substance.

“I think it’s going to work,” Merrill says. “It’s probably not going to work extremely well, because rats are extremely smart. They’ll watch their friend go in there, splash around a little and die, and then they won’t go in there.”

Merrill plans to retire next year. After a long career fighting the rodents, he says he has “respect” for them.

“I love to talk about rats!” he says. “I certainly don’t hate them, because without them, I wouldn’t have a job.”