Evidence gleaned from mobile phones and landlines will be central to the Soham murder trial, according to the prosecution.
Calls allegedly made by Mr Huntley will be used as evidence
Richard Latham QC claimed an electronic "goodbye" from Jessica Chapman's phone showed that she and Holly Wells could last be traced to a spot right outside the home of Ian Huntley - the man who denies their murder.
The jury also heard about a series of alleged phone calls between Mr Huntley and his ex-girlfriend Maxine Carr, who denies two charges of assisting an offender and one of conspiring to pervert the course of justice.
Mr Latham argued records from the calls and some text messages showed that she was in Grimsby when Mr Huntley allegedly killed the 10-year-olds and that she deliberately misled police.
As the jury gathered on Wednesday to hear evidence for the first time, Mr Latham told them: "To know where a
telephone was, or was not, can be very informative.
Prosecution's 'key phone numbers'
Jessica Chapman's Vodafone pay-as-you go mobile
Ian Huntley's mobile
A mobile owned by Soham Village College - where Mr Huntley worked
Three landlines at the school - its main number, community education office and kitchen
Maxine Carr's mobile number
The home number of Mr Huntley's father
"Telephone evidence, we say, is very important in this case."
He highlighted the fact that operators keep a record of all calls, and the spot where a mobile phone was used can be traced to within a short distance.
Mr Latham showed them a list of numbers he considered important and drew particular attention to the fact Jessica had a phone with her on Sunday, 4 August 2002, the day the girls disappeared.
The jury heard that as Jessica was about to leave for Holly's house, Rebecca, her "safety conscious" older sister, checked to make sure she had her pay-as-you-go mobile with her.
A final signal sent by Jessica's phone as it was turned off at 1846 BST, just 30 minutes after the best friends vanished, provided vital information, the prosecution said.
Mr Latham said it would have sent out the electronic signal if it was turned off, or ran out of battery power, as it "explicitly detached from the network".
The girls disappeared last year
The signal would have been sent automatically and was "nothing to do with human beings", he said.
By "a quirk" its last communication was sent to a telecoms mast in nearby Burwell, which could only be accessed from a handful of spots around Soham - including the area outside Mr Huntley's home, it was claimed.
Mr Latham suggested the girls were in the "immediate area" of the house, adding that the final signal from Jessica's phone would become "highly significant" as the jury heard further evidence.
Later on the Sunday night, at 2040 BST, Jessica's mother, Sharon, tried to call her daughter's phone after discovering the girls were missing. It was turned off.
Mr Latham also said phone records showed that Ms Carr was in Grimsby at the time he believes Mr Huntley killed Holly and Jessica.
He said Ms Carr made the trip on Saturday 3 August , the day before the girls disappeared, and only returned on Tuesday 6 August, because of what was happening.
The prosecution claims she only admitted being in Grimsby after her arrest and misled police about the times of calls between her and Mr Huntley while she was away and what was said in them.
In particular, it claims that Ms Carr said Mr Huntley told her that two girls had asked about her, but that he had been more interested in what she was doing that night.
The prosecution says the call was registered at 1824 BST - before Mr Huntley could have seen Holly and Jessica.