A man with the same refined tones as the gunman Anders Behring Breivik spoke calmly about shooting members of the youth wing of the Labour Party, said the radio station.
He also mentioned a manifesto during the phone call to the government in March 2011.
Due to the disturbing contents of the call, the receptionist detailed it on a written note, but this was never transmitted to police, the Norwegian government services centre said.
"The call was never considered as a real threat but more like a vague and incoherent conversation," Margot Vaagdal, head of the centre's communications, told AFP.
It was only after Behring Breivik's attacks that the police were alerted as the centre "found that a part of what was said was perhaps relevant for the case," said Vaagdal, although she would not confirm details of the call.
On July 22nd, the 32-year-old right-wing extremist first set off a car bomb outside government buildings in Oslo, killing eight people.
He then went to Utøya, some 40 kilometres north-west of Oslo, where, disguised as a police officer, he methodically shot and killed another 69 people attending a summer camp, most of them teenagers.
On the same day, he published on the internet a lengthy manifesto in which he expounded his Islamophobic and anti-multicultural views.
According to NRK, the receptionist had detailed the nature of threats, the name of the caller, his phone number and date.
The government services centre said it did not know if the caller was Behring Breivik himself, and that the note was somewhere in one of the buildings hit by the extremist's bomb.
Meanwhile, newspaper Aftenposten said that the gunman rang a ministry in May or June 2010 to obtain a list of the movements of different political parties' youth wings.
Quoting details from a police hearing of an official from the ministry, the newspaper said that the information requested by Behring Breivik was not released to him as the ministry did not have such details.
Court-appointed psychiatrists have concluded that Behring Breivik was criminally insane and therefore not accountable for his actions.
He is currently being held under provisional detention while awaiting the trial which opens on April 16th.