He told a court he had borrowed the illegal device from another inmate so he could keep in touch with his mum Ibby.
The consecutive sentence – added to his minimum 17-year life sentence for the “cold blooded” murder of his wife Arlene – means his earliest date for parole will now be 2030.
Fraser, 59, from Elgin, Moray, has previously told his visitors that one of the things he missed most in prison was his mother’s home-made soup.
After visiting him at Christmas time last year, a friend wrote on his Facebook page: “We spoke of when he was wee and his mammy made tattie soup with oat cakes.
“There’s a theme here. Nat it seems likes hame-made soup lol. So all in all he is trying to keep upbeat.”
Fraser, who appeared in the dock at Livingston Sheriff Court casually dressed in a grey Adidas T-shirt and denim jeans, gave a wry smile as he was told his jail term was being extended.
Handcuffed to a female security guard, he confirmed he was pleading guilty to having the prohibited phone in HMP Addiewell, West Lothian, on 7 June 2017. He also admitted having a previous conviction for an identical offence.
Rebecca Swansey, prosecuting, said prison officers had been sent to Fraser’s cell at 9pm on the day of the offence to carry out a “targeted search”.
She said: “On entering the cell the accused was lying on his bed with his hands under the covers.
“He was asked to how his hands and on doing so the witnesses could clearly see that he was holding a mobile phone.
“This mobile phone was seized and the matter was reported to the police.
“The accused was advised that matter would thereafter be reported to the procurator fiscal’s office.”
Iain Smith, defending, stressed that Fraser had not tried to hide the phone and had co-operated fully with prison staff.
He added: “He was placed before the Governor and was given a 14-day all-round loss of privileges as punishment.
“He borrowed the phone to telephone his mother. He’s not been operating some criminal activity or otherwise.
“He’s getting on with his sentence but accepts he’s done the wrong thing and that it’s for the second time.
“He’s normally a trusted prisoner and gained his trust back from the prison after doing his punishment.
“He’s a pass man in the long-term block and just keeps himself to himself.”
Mr Smith said Fraser had pled guilty at the earliest opportunity and didn’t wish to waste the court’s time by prolonging the case.
He added: “He apologises for appearing again before the court. He’s under no illusions about the sentence.”
Jailing Fraser, Sheriff Martin Edington told him: “As has been said many times in this court, possessing a mobile telephone in prison is a very serious matter and one that almost inevitably will lead to a prison sentence.
“That’s particularly so when the accused is, as you are, already serving a custodial sentence.
“In addition, you have a directly analogous previous conviction and you knew exactly what you were doing and you also knew you should not have been doing it.
“I’ve come to the conclusion that there is no alternative in the public interest but to impose a custodial sentence.
“In view of your previous conviction that sentence will be nine months imprisonment, reduced to six months in view of your immediate acceptance of your guilt.
“It will, however, be consecutive to your prison sentence, not concurrent.”
Fraser was convicted twice of organising the killing of his 33-year-old wife Arlene after she made plans to divorce him.
She vanished from their home on 28 April 1998 after her two children – then aged 20 and five years – went to school.
No trace of her was ever found.
His friend Hector Dick, who was originally charged with conspiracy to murder Arlene, told the trial that Arlene’s husband had hired a hitman to kill her, and then burned her body and ground up the remains.
Nat Fraser was found guilty of her murder in January 2003 and sentenced to life imprisonment, with a minimum of 25 years. He collapsed in the dock as the judge passed sentence.
In May 2006, he was released from prison pending the outcome of his appeal amid claims that relevant evidence had not been made available at his trial.
He was returned to prison in December 2007 and the appeal was refused on 6 May 2008.
In May 2011, after Fraser won a further appeal at the Supreme Court, judges quashed his conviction and the Crown Office immediately announced it would seek to bring fresh proceedings against him.
In 2012, Fraser again went on trial for murdering Arlene. He was once more found guilty and sentenced to a minimum 17-year life term.
The second trial was the subject of an award winning television documentary “The Murder Trial”, which was shown on Channel 4 on 9 July 2013.
Fraser and his supporters still maintain that he is the victim of a miscarriage of justice and that he did not murder his wife.
However, an appeal by him against the second murder conviction was refused in October 2013.