Giving his east London address, Stephen Port said about 23-year-old Anthony Walgate: "Appears a young boy looks like he's collapsed outside."
Mr Port then went to sleep rather than wait for paramedics, the jury heard.
He denies 29 charges including four counts of murder, as well as drugging, rape and sexual assault.
The 41-year-old chef allegedly plied his four young murder victims with the drug GHB at his Barking home so he could have sex with them while they were unconscious.
The Old Bailey trial has heard that Mr Port was jailed for eight months for perverting the course of justice by lying to police about the circumstances of the death of Mr Walgate, a fashion student.
They had made contact online in June 2014 and Mr Walgate agreed to go to the chef's home for a sleepover, the court heard.
Mr Walgate had told friends he was going to meet a man named Joe Dean in Barking who was going to pay Â£800 to spend the night with him.
His university friends said the photo they were shown of "Joe Dean" was the man they now know as Stephen Port, the jury was told.
In the 999 call played in court, Mr Port told the operator: "Looks like he has collapsed or had a seizure or something - or just drunk."
The accused was no longer on the line when he was asked for his number, and the operator had to call Mr Port back to ask if the person was breathing.
In a statement, paramedic Anthony Neil said he arrived to find Mr Walgate was already dead and was "in a sitting position against something".
He covered him in a red blanket and alerted police, the court heard.
Police officers tracked Mr Port down to his flat through his number, noting him as a witness and took his statement.
The jury has heard how officers later realised Mr Port had hired Mr Walgate as an escort and had been lying about the events of that night.
Pathologist Olaf Biedrzycki told the court the student's death was due to GHB intoxication and said he had been informed that Mr Walgate's jeans zip was undone and his underwear was "inside out and back to front".
There were also 14 separate injuries on the body including grazes and bruising to the inside upper left arm.
Prosecutor Jonathan Rees QC told the court that the second alleged murder victim, Gabriel Kovari, had been Mr Port's temporary flatmate.
The court heard how, before moving in, the Slovakian national had reassured a friend who asked him: "Are you sure he's safe? There are some crazy people out there lol."
Mr Kovari's body was found by a dog walker in a churchyard near Mr Port's home on 28 August 2014.
The trial continues.
An alleged serial killer's 999 call to report that he had found a man - said to be his first murder victim - outside his flat has been played to a jury.