That call from the South Korea ferry Sewol was forwarded to the coastguard two minutes later and was followed by about 20 others by phone from children to the fire brigade, a fire station officer told Reuters.
The Sewol ferry sank last Wednesday on a routine trip south from the port of Incheon to the traditional honeymoon island of Jeju.
Of the 476 passengers and crew on board, 339 were children and teachers on a high school outing. Only 174 people have been rescued and the remainder have been added to the death toll.
The boy who made the call, with the family name of Choi, is among the missing. His voice was shaking and sounded urgent, a fire officer told MBC TV. It took a while to identify the ship as the Sewol.
"Save us! We're on a ship and I think it's sinking," Yonhap news agency quoted him as saying.
The fire station official asked him to switch the phone to the captain, and the boy replied: "Do you mean teacher"
The pronunciation of the words for "captain" and "teacher" is similar in Korean.
The captain of the ship, Lee Joon-seok, 69, and other crew members have been arrested on negligence charges. Lee was also charged with undertaking an "excessive change of course without slowing down".
ONLY OBEYING ORDERS
Several crew members, including the captain, left the ferry as it was sinking, witnesses have said, after passengers were told to stay in their cabins. President Park Geun-hye said on Monday that instruction was tantamount to an "act of murder".
Many of the children did not question their elders, as is customary in hierarchical Korean society. They paid for their obedience with their lives.
Public broadcaster KBS, quoting transcripts of the conversation between the crew and sea traffic control, the Jindo Vessel Traffic Services Centre, said the passengers were told repeatedly to stay put.
For half an hour, the crew on the third deck kept asking the bridge by walkie-talkie whether or not they should make the order to abandon ship, KBS said.
No one answered.
"At the time, we could not confirm what the situation was on the bridge," KBS quoted a crew member as saying.
"We kept trying to find out but ... since there was no instruction coming from the bridge, the crew on the third floor followed the instructions on the manual and kept making 'stay where you are' announcements. At least three times."
Lee was not on the bridge when the ship turned. Navigation was in the hands of a 26-year old third mate who was in charge for the first time on that part of the journey, according to crew members.
The government on Tuesday released a timeline of the ferry's last minutes, stable and afloat. It made its scheduled turn en route to Jeju at 8.49 a.m. and started listing within a minute. By 8.55 a.m. it was drifting back on itself and keeling to port.
Lee, seen on television with his head lowered and covered, told reporters soon after the sinking he feared passengers would be swept away by the ferocious currents if they abandoned ship. He has not explained why he left the vessel.
In a confused exchange between the sinking Sewol and maritime traffic control released by the government, the crew said the ship was listing to port.
"Make passengers wear life jackets and get ready in case you need to abandon ship," traffic control said.
The Sewol answered: "It's difficult for the passengers to move now."