Joseph Luke Forester-Watson rang the woman and told her: ‘I’m thinking of ways of getting payback on you, because you called the police.
‘I know it was you.’
The 18-year-old will be sentenced in summary court on March 11 after a probation report has been prepared.
Prosecuting advocate Victoria Kinrade told the court that Forester-Watson had been arrested in relation to another matter, on January 12, but had been granted bail by the police.
On January 21, the woman who had reported him received a call, from a withheld number, during which Forester-Watson made the comment.
She recognised his voice and said that she was concerned for herself and her family.
Forester-Watson then made a second call, apologising for his previous comments.
He was arrested and during an interview with police, admitted making the calls.
He said he had been angry, and had had thoughts about stabbing the woman with scissors or a knife.
Forester-Watson said that the probability of him doing this was around four out of 10, but if he was charged, he said it would increase to nine or 10 out of 10.
He told police he would feel happy once he had stabbed her, and that he wanted to hurt her, to stop her giving evidence.
Defence advocate Paul Glover said that, although his client had withheld his number on the first occasion, he had identified himself.
Deputy High Bailiff Rachael Braidwood accepted summary court jurisdiction, saying that, had the things Forester-Watson said in interview been said directly to the woman, she would have sent the case to the Court of General Gaol Delivery.
No bail application was made and Forester-Watson, who lives at Circular Road, is remanded at the prison.