Whatever happened to Laura Babcock the night of July 3, 2012, her phone was still alive the next day.

Although the last outgoing call was made at 7:03 p.m. that night, the 23 year-old's BlackBerry continued to receive calls and texts the morning of July 4.

Since that activity was recorded by Babcock's cellular provider, a small but now critical amount of information is known about it.

It's information the Crown is attempting to use, more than five years later, to prove the whereabouts of Babcock and the two men accused of killing her.

Dellen Millard, 32, of Toronto and Mark Smich, 30, of Oakville, Ont., are both pleading not guilty to first-degree murder.

Most cell providers log the phone numbers of incoming and outgoing calls and texts, the time and date they were made, the duration of calls, and, importantly, the approximate location of the phone when this activity took place.

What's actually recorded is the location of the "cell site" that a phone connected with in order to place or receive a call or text. These can be, for example, cellular towers or antennas mounted on buildings.

Since the range of each cell site is limited to a certain distance and a phone will often connect to the nearest one, police and prosecutors use this information to prove someone's whereabouts.

Or at least they try to.

The final call

According to "call detail records" submitted as evidence, the 7:03 p.m. call was 60 seconds in duration and likely made to retrieve Babcock's voicemail.

The phone connected with a cell site at 210 Markland Drive in Etobicoke — approximately 400 metres as the crow flies from Dellen Millard's home at 5 Maple Gate Court.

Using phone records, the Crown is arguing that Babcock, Millard, and Smich were all at or near that address on July 3, 2012.

Dellen Millard, left, and Mark Smich, right, are accused of first-degree murder in the death of Laura Babcock. (Facebook, Instagram)

'Like a tracking device'

On Thursday, as Justice Michael Code prepped the jury for the evidence, he described a cellphone as being "almost like a tracking device."

Police know this. Since people now have phones with them nearly everywhere they go, investigators can use them, in cases like these, to figure out where they went.

Joseph Sadoun, a director for the telecommunications engineering firm Yves R. Hamel and Associates, describes cellular data records as "footprints" phones leave behind — but not everywhere.

Joseph Sadoun, director of engineering for the telecommunications engineering firm Yves R. Hamel and Associates, says call detail records can't pinpoint exactly where a phone was. (CBC)

The "tracking device" metaphor isn't quite accurate, Sadoun says, because providers have no way of knowing where a phone is when it's not in use, unless police request special, court-approved surveillance.

But once a call or text is made or received, the phone connects with the cell site.

"At that moment, the network knows where the phone is," Sadoun said in an interview.

But he admits it's not "pinpoint" accurate. Sadoun says the records can only show a phone was in the range of the cell site it connected with.

It's a limitation in the technology that defence lawyers and experts have sought to exploit.

Closest site or clearest site?

"Scientifically, it's not quite correct," Joseph Kennedy, a senior manager with Cherry Biometrics, said in an interview.

The Virginia-based firm specializes in cellular communications analysis. Kennedy has been called as a defence witness in criminal trials to contest call detail record evidence that attempts to prove a person's whereabouts.

"You only know an area, a rather large area, where the phone could possibly be," Kennedy said of this kind of evidence. "That's all we can scientifically state with confidence."

Kennedy says there are several factors that determine what cell site a phone will connect with, and distance is just one of them.

Adnan Syed, convicted in 2000 of killing Baltimore high school student Hae Min Lee, will have a new trial because his conviction relied heavily on call detail records. (Carlos Barria/Reuters)

The topography of an area, obstruction from buildings, and the weather are all things that could cause a phone to connect with another site.

Kennedy also says if the nearest cell site is too busy with other calls, the network will connect a phone to another site that could be further away.

"It's not going to go to the closest tower, it's going to go to the clearest tower," he said.

Evidence questioned

Kennedy says this type of evidence has led to many convictions in the United States but it doesn't always hold up.

His colleague, Michael Cherry, worked with the defence in the case of Lisa Marie Roberts of Portland, Ore.

Roberts spent more than a decade in prison before her manslaughter conviction was overturned in 2014. The original decision was based in part on the misinterpretation of a cellphone call.

Dellen Millard, 32, is acting as his own lawyer at the Laura Babcock murder trial. Co-accused Mark Smich has a lawyer. (Toronto Police Service)

While Roberts' phone did connect to a site near the park where the victim's body was found, it was later determined that she made the call while driving more than 12 kilometres away and the cell network happened to route her call to another site.

Last year, a new trial was ordered for Adnan Syed, who was convicted in 2000 of killing Baltimore high school student Hae Min Lee.

The case, which was profiled in the 2014 podcast Serial, relied heavily on call detail records and the location of cell sites.

A judge vacated Syed's conviction over questions about the reliability of the cell phone evidence.

July 4: Contact lost

On the morning of July 4, 2012, Laura Babcock's phone was functioning, receiving several unanswered phone calls and text messages.

Thanks to these calls and texts, the phone continued to connect with cell sites. And based on that information, the allegation is that Babcock's phone was on the move.

Together with Millard's phone, the Crown said in its opening statement that Babcock's BlackBerry travelled from Millard's home in Etobicoke westward along Lake Ontario.

According to the Crown, Babcock's phone stopped connecting with cell sites shortly after 11 a.m., somewhere along this route.

It's evidence the Crown is asking the jury to consider alongside the rap song, written and performed by Mark Smich, that concludes with the line:

"If you go swimming you can find her phone."

What the jury will make of this, remains to be seen.