When a call came in from her sibling’s cellphone days after she went missing, Amanda Barthelemy didn’t know what to think.

“Is this Melissa’s little sister?” a man asked.

“Yes,” replied the 16-year-old girl.

“Do you know what your sister is doing?” he said. “She’s a whore.”

The call, police believe, was made by a sadistic serial killer who abducted and strangled Melissa Barthelemy, an aspiring hair stylist, sometime after she left her Bronx apartment on July 10, 2009.

But not before extracting a secret from the 24-year-old victim, who moved to New York from Buffalo in 2007.

Melissa had not told her mother she was turning tricks to pay her bills, though she’d been booking clients on Craigslist and meeting johns for sex for more than a year.

She lied and said she was an exotic dancer. Only Amanda got the truth.

“No one else knew,” said their mom, Lynn Barthelemy.

It was the first real clue about the twisted psyche of a predator who stalked and choked to death at least eight prostitutes in the last five years, and possibly murdered four others.

That’s the body count after cops this week linked the murder of Melissa Barthelemy and three other prostitutes found near Gilgo Beach in Suffolk County in December to four dead sex workers left in a ditch near Atlantic City in 2006, sources told The Post. All died from strangulation, their corpses stashed before being dumped.

“It’s the same guy,” said one law-enforcement source.

A third cluster of four bodies discovered a few miles from Gilgo Beach last week has fueled speculation that a Long Island serial killer used the sandy south shore as his burial ground.

There are conflicting reports about a possible suspect. Suffolk officials said they don’t have one, but a police source familiar with the investigation said cops are focusing on a man who lives in the county. They’ve been watching him for days, the source said.

And they’re re-examining every bit of evidence in the cases, including that first call to Amanda, which was followed by a half-dozen more during the next six weeks.

The killer always phoned in the evenings, spoke briefly and in a low voice — and only to Amanda. He calmly spewed taunts and allegations. He sent text messages.

“He wouldn’t talk to anybody but her,” said Lynn Barthelemy, who documented the barrage in a journal she gave police. “One time I answered, and he hung up as soon as he heard my voice.”

Cops began to triangulate the phone, pinpointing the caller’s location to Times Square, Madison Square Garden and Massapequa, a short drive from Gilgo Beach. They showed Melissa’s picture around at strip clubs. Did her murderer work in Midtown and commute from Long Island?

The girls were close, and their family wondered: Had the killer gotten a glimpse of Amanda, who twice came to stay with Melissa?

“When Amanda went down there, the first thing they did was to get a manicure and pedicure,” said their mom. “They went to the zoo and the Statue of Liberty.”

But the sister did not recall anyone who might have been a suspect.

The calls abruptly stopped in August 2009 after a Buffalo TV station revealed their existence.

But by then, police believe, Melissa was already dead. The stories of others would soon emerge.

When Megan Water man went on the road to meet her clients, she’d drop off her 4-year-old daughter, Lily, with her grandparents, borrow a friend’s Ford Explorer or catch the bus from Scarborough, Maine, to Long Island.

She would go at the request of her boyfriend, Akeem Cruz, who called himself “Vibe” and doubled as her pimp. He used a laptop to place ads for her services and accompanied her to the Holiday Inn Express in Hauppauge. While Waterman, 22, met johns, Cruz visited his family nearby.

“I told her I thought it was so crazy and dangerous,” said her friend Nicole “Nicci” Haycock, who dates Waterman’s brother Greg. “But Megan was very free.”

The couple took a bus to Long Island last spring and checked in at the hotel on June 5. That night, they left together at 8 p.m., and she returned 30 minutes later by herself. At 1:30 a.m., she got a call on her cell from Cruz’s cell, then left her room alone.

A witness last spotted Waterman outside, walking toward a convenience store, Haycock said. It was the last time Waterman was seen alive.

Waterman’s body was the first to be identified from among the four remains found near Gilgo Beach. Cops grilled Cruz. About three weeks ago, they seized his laptop.

“He’s not considered a suspect, but he hasn’t cooperated,” said Haycock.

Maureen Brainard-Barnes, the earliest Long Island victim, was an energetic single mom of two young daughters who wrote poetry and liked the books of Shel Silverstein.

The 25-year-old brunette came into the city by train with two acquaintances, Brett and Sarah, from Norwich, Conn., on the morning of July 9, 2007, to meet clients, a friend said.

Brainard-Barnes called her friend in distress that night from the Port Authority Bus Terminal, saying she had been robbed of the money she earned that day.

“She just wanted to come home,” said the friend. “She said she was coming that night and would check in.”

Brett and Sarah returned without Maureen, who had gone off by herself.

“They said they didn’t know what happened to her,” the friend said.

Amber Lynn Costello, 27, who vanished from Long Island on Sept. 2, was the killer’s most recent victim.

A petite woman who stood under 5 feet and weighed only 100 pounds, Costello grew up in Wilmington, NC, where she was physically abused as a youth, according to a friend who asked not to be identified.

The abuse led to drug addiction, which she supported by working for Private Playmates escort service.

After getting divorced, she moved to North Babylon last year and continued her spiral of drugs and prostitution.

The victims might never have been dis- covered had it not been for the disappearance of Shannan Gilbert, a 24-year-old aspiring singer from Jersey City who, like the others, booked johns through Craigslist.

At 2 a.m. on May 1, Gilbert’s driver took her to the home of customer Joe Brewer in the gated, millionaires’ community of Oak Beach, a few miles east of Gilgo Beach, and waited outside. At 5 a.m., Brewer called him and complained that Gilbert would not leave.

The driver went in and found Gilbert dressed, “delirious” and on the phone with 911. She said to him, “You guys are trying to kill me” and she ran from the house, the driver said. He followed.

She pounded on the door of neighbor Gus Coletti, saying, “Help me. Help me.” Coletti thought she was high but unhurt. She ran off.

The driver returned the next day with Gilbert’s boyfriend to continue their search.

Police combed Brewer’s house soon after Gilbert disappeared, and their search for her led them to the first cluster of four bodies near Gilgo Beach.

They again searched Brewer’s home following last week’s discovery of four more bodies. But they found nothing to tie him to the murders.

Police sources would not say what led authorities to link the New Jersey murder victims — Kim Raffo, Molly Jean Dilts, Barbara Breidor and Tracy Ann Roberts — to the first four in Long Island. But there were several common elements.

In each case, the killer dumped four prostitutes near water and in close proximity, tossing their remains off remote, desolate sections of highway. The bodies were in various stages of decomposition, suggesting he kept some for a period of time after they died.

All eight were low-level sex workers, and the four in New York used Craigslist. Each had been strangled.

He had removed the shoes from both sets of bodies, though the women in New Jersey were clothed. Those in Long Island had been stripped naked and were found without jewelry or belongings, each wrapped in burlap.