The city promised a lifeline for sex-trafficking victims — but gave them a wrong number instead.

First lady Chirlane McCray proudly unveiled a 24-hour trafficking tip line at a 2017 press conference alongside the police commissioner, claiming it will connect callers with a “specially trained officer” and bring “justice to . . . traffickers.”

But a call to the number that McCray pointed to during her photo op reached an entirely different unit. “No, this is not the sex-trafficking hotline. How can I help you?” asked the woman who answered the 646-610-7272 “NYPD Trafficking Hotline.”

The number is actually for the Special Victims Division, which doesn’t handle sex-trafficking cases. After conferring with colleagues, she offered a number for the Vice Unit, which is responsible for those cases.

But the woman who answered in vice also said that number wasn’t the hotline and said to call the original 7272 number.

The Post discovered the number blunder this week amid a series of exposés on the scourge of sex trafficking in New York City.

Thousands of victims — many as young as 12 — are being sold for sex by ruthless pimps across the five boroughs every day.

It’s the exact problem McCray and Police Commissioner James O’Neill said they were tackling when they unveiled the “hotline” in February 2017 — alongside an extra 25 detectives dedicated to sex trafficking. “We’re starting a new hotline, which any victim of trafficking can call for help and any person who witnesses trafficking can report it. The number is 646-610-7272,” the first lady said at the time, standing alongside a trafficking survivor.

“A specially trained officer will answer, and every single report will be investigated by human-trafficking enforcement. And to make sure everyone knows about this number, we will spread the word through an advertising campaign focusing on those communities that are highest at risk.”

She advertised the number again this year in a Facebook video on Jan. 11 — National Human Trafficking Awareness Day — and several NYPD precincts tweeted out the digits.

The bungle is a huge disservice to sex-trafficking victims, said Julie Laurence, chief program officer at Girls Educational & Mentoring Services.

“It is a big risk to trafficking victims to call out for help, and we need to ensure that any call made is responded to quickly and with compassion and care and viable options for support and services,” she said.

And the number McCray announced isn’t the only hotline with issues. The first result of a Google search for “NYPD sex trafficking hotline” and similar queries is an NYC.gov page called “Let’s End Human Trafficking.” That directs tipsters to a hotline run by the NYPD’s Organized Crime Control Bureau — which was disbanded years ago.

The number now goes through to a drug hotline. The woman who answered there did offer to take a complaint “if someone’s in trouble.”

The Brooklyn District Attorney’s Office promotes a 24-hour line, but one call there was answered by a befuddled cop who said, “Sex trafficking? You got me there.” He then transferred the caller to the office’s human-trafficking unit. A woman who answered there was annoyed the cop had passed off the call but was willing to help.

A second attempt went better, with a woman answering and affirming that it was indeed the sex-trafficking hotline.

The man who picked up on the Manhattan DA’s hotline was the only one who actually answered by saying, “Human-trafficking hotline.”

But that line is only manned during regular business hours. After-hours calls go through to voicemail that is checked the next morning, a staffer said.

No other district attorneys advertise dedicated sex-trafficking hotlines on their sites.