A government switchboard operator noted the conversation in which an unnamed caller spoke of shooting youth members of the ruling Labour Party.
The caller, whose voice resembled that of Breivik, also talked of a manifesto.
Police were not informed because the threat was not considered to be serious at the time, an official said.
"The call was never considered as a real threat but more like a vague and incoherent conversation," Margot Vaagdal, communications chief at the government services centre, told AFP news agency.
Breivik has admitted twin attacks on 22 July in which he killed 77 people and injured 151, traumatising the small north European nation.
He set off a bomb in the capital, Oslo, before travelling to the lake island of Utoeya where he shot young Labour Party activists attending a summer camp.
On the same day, he published on the internet a lengthy manifesto in which he outlined his hostility to Muslim immigration and multi-culturalism.
The call uncovered by public radio was made some time in March.
It has also emerged that Breivik phoned a government ministry in May or June 2010 to obtain membership lists for various political parties' youth wings.
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Anders Behring Breivik
Describes himself as a Christian and conservative on Facebook page attributed to him
Grew up in Oslo and attended Oslo School of Management
Set up farm through which he was able to buy fertiliser, which may have been used to make a bomb
Profile: Anders Behring Breivik
Manifesto details attacks
The information was not released to him as the ministry did not have such details, public radio reports.
Breivik is in custody awaiting trial on 16 April.
Experts monitoring him said this week they believed he was not psychotic, contradicting court-appointed psychiatrists.
The court-appointed psychiatrists had declared him insane after interviewing him on 13 occasions.
But the expert team of four psychiatrists assessing Breivik in prison disagreed with several of the original conclusions.
According to the report submitted by the Public Prosecutor, Svein Holden, they do not believe Breivik is psychotic or schizophrenic and do not think he needs drugs.
In addition they do not regard him as being at high risk of committing suicide.