The athlete, who was found guilty of culpable homicide but cleared of murder in Pretoria last month, apparently spoke to fellow Olympian Jenna Edkins for nine minutes on Valentine’s Day in 2013.
The conversation was not included as evidence during his six-month trial because police were apparently unaware it had ever existed, according to the book.
The number used by Edkins had supposedly been registered in her father’s name and was saved in Pistorius’ contacts as “Babyshoes”.
It has been claimed that officers investigating Steenkamp’s death had failed to make the connection between the name “Babyshoes” and Edkins.
Oscar Pistorius: Career in pictures
Oscar Pistorius: Career in pictures
1/34 Oscar Pistorius in Greece
Oscar Pistorius wins the 200m T44 for Men during the Athens 2004 Paralympic Games at the Olympic Stadium in Athens, 2004
2/34 Oscar Pistorius in Greece
Oscar Pistorius at the Olympic Stadium in Athens, 2004
3/34 Oscar Pistorius in Manchester
Oscar Pistorius celebrates winning his second gold medal during the VISA Paralympic World Cup Athletics at the Sport City Stadium in Manchester, 2006
4/34 Oscar Pistorius in Manchester
Oscar Pistorius shows his gold medal after winning the T44 200 m for men at the Visa Paralympic world cup at the Manchester Regional Arena, 2007
5/34 Oscar Pistorius competes in race
Oscar Pistorius competing in the men's 400m race B, 2007
6/34 Oscar Pistorius in Rome
Oscar Pistorius settles in the blocks before winning the Mens 400m 'B' race during the IAAF Golden Gala at The Olympic Stadium in Rome, 2007
7/34 Oscar Pistorius in Rome
Oscar Pistorius poses during the broadcast 'Piazza grande' on Italian TV state channel 'Rai 1' in Rome, 2008
8/34 Oscar Pistorius in St.Petersburg
(L to R) Bryan Habana, Cuba Gooding Jr and Oscar Pistorius during the Laureus welcome reception at Gimnazya in St.Petersburg, 2008
9/34 Oscar Pistorius arrives at the Court of Arbitration for Sport (CAS) in Lausanne
Oscar Pistorius arrives at the Court of Arbitration for Sport (CAS) in Lausanne to appeal his case on the second day of his hearing as he was told by the International Association of Athletics Federations (IAAF) that he could not compete in their events because the artificial legs he uses gives him an unfair advantage, 2008
10/34 Oscar Pistorius runs during a training session
Oscar Pistorius runs during a training session on the track of Emmeloord, 2008
11/34 Oscar Pistorius in Germany
Oscar Pistorius compete during the 400 meter race at the German disabled athletics championships in Berlin, 2008
12/34 Oscar Pistorius in Italy
Oscar Pistorius of South Africa relaxes prior to the 400m race in his quest to make the Olympic qualifying time of 45.55 during the Notturna di Milano International Athletics Meeting at the Arena Gianni Brera in Milan, 2008
13/34 Oscar Pistorius in China
Oscar Pistorius speaks at a press conference for the 2008 Beijing Paralympic Games in Beijing, 2008
14/34 Oscar Pistorius in China
Oscar Pistorius of South Africa celebrates after winning the gold medal in the Men's 100m T44 Final Athletics event at the National Stadium during day three of the 2008 Paralympic Games in Beijing, 2008
15/34 Oscar Pistorius in China
Oscar Pistorious (L) leads Heros Marai (C) of Italy and Christoph Bausch (R) of Switzerland in the heats of the men's 100 metre T44 classification race at the 2008 Beijing Paralympic Games in Beijing, 2008
16/34 Oscar Pistorius and Nelson Mandela in South Africa
Oscar Pistorius and former South African President, Nelson Mandela in South Africa, 2009
17/34 Oscar Pistorius in Norway
Oscar Pistorius looks on prior to the 400m B-final during the IAAF Golden League Bislett Games in Oslo, 2009
18/34 Oscar Pistorius in London
Oscar Pistorius reacts after winning the men's 400 metres T44 final race with a new world record time of 47.04 sec during the Diamond League athletics meeting at Crystal Palace in London, 2010
19/34 Oscar Pistorius in New Zealand
Oscar Pistorius of South Africa celebrates receiving his gold medal after winning the Men's 200m T44 final during day three of the IPC Athletics Championships at QE II Park in Christchurch, New Zealand, 2011
20/34 Oscar Pistorius in New Zealand
(L-R) Arnu Fourie, Henry Roos, Oscar Pistorius and Mike Samkelo Radebe of South Africa celebrate after winning the Men's 4x100m Relay T42-46 final during day eight of the IPC Athletics Championships at QE II Park in Christchurch, New Zealand, 2011
21/34 Oscar Pistorius in New Zealand
Oscar Pistorius of South Africa poses with his medals during day eight of the IPC Athletics Championships at QE II Park in Christchurch, New Zealand, 2011
22/34 Oscar Pistorius in South Africa
Oscar Pistorius in action during the 400m during day 5 of The Nedbank National Championships for the Physically Disabled from Olympia Park Stadium in Rustenburg, 2011
23/34 Oscar Pistorius in Czech Republic
Oscar Pistorius warms up before the men's 400m event at the Zlata Tretra (Golden Spike) athletics meeting in the eastern Czech city of Ostrava, 2011
24/34 Oscar Pistorius in New York
Oscar Pistorius attends the Eva Mendes launch of her new campaign for Angel by Thierry Mugler at IAC Building in New York, 2011
25/34 Oscar Pistorius in South Korea
Oscar Pistorius of South Africa speaks with Olympic gold medalist Michael Johnson of the United States during a media gathering at the Novatel Hotel prior to the 13th IAAF World Athletics Championships Daegu 2011 in Daegu, South Korea, 2011
26/34 Oscar Pistorius in South Korea
Oscar Pistorius prepares to settle into the blocks to compete in the men's 400 metres heats at the International Association of Athletics Federations (IAAF) World Championships in Daegu, 2011
27/34 Oscar Pistorius in London
South African sprint runner known as 'Blade Runner' Oscar Pistorius (C) poses for photographers with Paralympians ahead of the international Paralympic Day in central London, 2011
28/34 Oscar Pistorius in London
Oscar Pistorius poses in the press room with his Laureus World Sportsperson of the Year with a Disability trophy at the 2012 Laureus World Sports Awards at Queen Elizabeth II Conference Centre in London, 2012
29/34 Oscar Pistorius in London
Oscar Pistorius of South Africa competes in the Men's 400m Round 1 heat on Day 8 of the 2012 London Olympic Games at the Olympic Stadium in London, 2012
30/34 Oscar Pistorius receives his honorary doctorate
Oscar Pistorius holds his scroll after receiving his honorary doctorate from Strathclyde University in the Barony Hall in Glasgow, 2012
31/34 Oscar Pistorius with his girlfriend
Oscar Pistorius posing next to his girlfriend Reeva Steenkamp at Melrose Arch in Johannesburg, 2013
32/34 Oscar Pistorius in court
Oscar Pistorius hides his face in his hands in the court room during his hearing on charge of murdering his model girlfriend Reeva Steenkamp at the Magistrate Court in Pretoria, 2013
33/34 Oscar Pistorius in court
Oscar Pistorius stands in the Magistrate Court in Pretoria after appearing for the first time since being freed on bail over the Valentine's Day killing of his model girlfriend, 2013
34/34 Oscar Pistorius in court
Oscar Pistorius appeared on charges of murdering his model girlfriend Reeva Steenkamp, 2013
The phone on which Pistorius called Edkins was one of two handsets, and was apparently removed from the crime scene for almost two weeks before it was handed over to the authorities.
By the time it was given in, its entire call history had allegedly been wiped.
As a result, Edkins was never questioned by the authorities and therefore did not provide a statement.
The apparent addition to Pistorius' story is set to be published in Behind the Door – a book about the trial by two leading journalists on the case, Barry Bateman of Eyewitness News and Mandy Wiener.
According to Bateman, the prosecution has so far declined to comment on how the evidence was missed.
#OscarPistorius handset’s entire call history and WhatsApp record was deleted, as well as several messages received after the shooting. BBBarry Bateman (@barrybateman) October 2, 2014
The Pistorius family said that they “were not aware of any deletions that could be relevant to this trial, or have impacted on this trial”.
Pistorius and Edkins apparently enjoyed a turbulent relationship dating back to 2008.
The day after news of Steenkamp’s death broke, Edkins posted the following on Twitter:
People must stop jumping on the bandwagon with such hurtful allegations. Os is the loving , amazing inspirational person we know him to be..Jenna Edkins (@JennaEdkins) February 15, 2013
You have all my familys love and support #loveandsupportoscarJenna Edkins (@JennaEdkins) February 15, 2013
All I am saying is let him speak , let his side be heard without jumping to conclusions.. Love and thoughts to Reevas famiy..Jenna Edkins (@JennaEdkins) February 15, 2013
I would just like to say , I have dated Oscar on off for 5 YEARS, NOT ONCE has he EVER lifted a finger to me , made me fear for my life..Jenna Edkins (@JennaEdkins) February 15, 2013
Pistorius was convicted of culpable homicide – the equivalent of manslaughter – on 12 September.
He is set to be sentenced on 13 October, and faces up to 15 years in jail.