For Frances Schulz, the mystery of what happened to her foster daughter Cherie Westell has been painful.

"Not knowing is full of anxiety,” Ms Schulz said.

The 15-year-old was days away from celebrating her 16th birthday when she went to the dentist in Wantirna South, in Melbourne's eastern suburbs, on December 12 2000.

She attended the appointment, then walked to a phone box to call home.

That was the last time anyone heard from her.

"Times like this are very emotional. I well up thinking about it, thinking about Cherie and not knowing what happened to her. Someone had to see her, someone had to talk to her,” Ms Schulz said.

Cherie was a cheeky teenager. Ms Schulz described her as "fiesty, but a typical teenager".

She loved horses, sport, art and music. Her favourite performer was Jewel. She'd spend hours listening to her songs.

As a child, the 15-year-old loved the monkey bars. She would often be found playing on them upside down, as Ms Schulz fondly recalled: "I can actually still remember holding her hand as little girl and the callous' on her hands."

But she had never run away.

And that's why her disappearance baffled her family.

"When a body is found police will get in touch and say ‘it's not her’… part of me feels I would like to know that it is her,” Ms Schulz said.

“The other part says as long as we don't know, there's still that little glimmer (of hope) which I try to hold on to."

But there were complexities with Cherie's case. She was a ward of the state in the care of another woman, and when Ms Schulz tried to make a Missing Persons report, she was told it had to be done by a family member.

That task was given her older brother, Pierre, who was on Schoolies in Queensland at the time.

Despite the early hours being crucial in missing persons investigations, the Missing Persons report wasn't filed until six days after her disappearance.

It was an issue that angered Ms Schulz.

"This is not just about Cherie. We feel we won't see her again but there are other families out there the most important thing to say is if someone is missing please do something immediately, don't wait,” she told 9NEWS.

“Don't be fobbed off by being told 'tomorrow, comeback again'. Be insistent, make sure that a report is made, a formal Missing Persons report and the police take action."

A coroner eventually ruled that Cherie had most likely died, but could not determine how or why.

The report was highly critical of the lack of communication between government and non-government agencies.

But that was little comfort for Ms Schulz, who has now accepted the girl she cared for is no longer alive.

In fact, she's convinced the teenager was murdered.

"There must be at least one person apart from Cherie who knows what happened to her,” she said.

“We're convinced Cherie met with foul play. So, we would like someone to come forward."

Police keep in contact with Ms Schulz. When Boronia schoolgirl Bung Siriboon vanished, Cherie's case became a detail in the investigation.

The pair went to the same school, prompting police to investigate if there was any link between the two cases.

"We know what Bung's family went through. She vanished without a trace,” Ms Schulz said.

Seventeen years since Cherie vanished, Ms Schulz has moved away from Melbourne.

Cherie would now be 32. Her brother is also grown up and has a family of his own. A family that misses a sister, an aunty, and for Ms Schulz, a daughter.

"It takes your energy emotionally, physically wand it gets less in that you do have to go on. But Cherie's still there. She's always part of it and she always will be," Ms Schulz said.

Ms Schulz still lives with the pain. All she wants is to know what happened.

"That's the most difficult part is not knowing what happened to Cherie. If we had answers we could at least, say goodbye to her."