The call was recorded on phone records as coming from Hume's cellphone, and lasted 11 seconds.
The complainant gave evidence that Hume sounded angry, like he was making the statement through gritted teeth.
Her body went into "complete meltdown" following the call, and she described herself as "absolutely terrified", shaking and speechless.
"I've never experienced anything like that in my life."
The complainant called police, who were in attendance when Hume and his two children appeared outside the offices of his former workplace around an hour later, carrying placards, one of which read "[company] stole $30,000 from us".
The attending officer gave evidence that he asked Hume to move on, and he complied.
He was arrested at his home the following evening.
Hume vehemently denied the allegations, saying all he said on the phone was "I'll speak to my girlfriend, thanks," at which the complainant hung up.
He said it was his children's idea to make the placards, and alleged his former employers owed him money so he was going to protest outside their workplace, having been trespassed from the property.
He also contacted the Wanganui Chronicle in the hopes of getting media coverage of his protest, and causing embarrassment to the company.
He said he called his former workplace to warn his then-girlfriend of the protest.
Hume said it was not until he spoke to the officer there that he learned of the allegation of threats. He claimed to be completely shocked, and called the situation a "set-up", saying his former employers were trying to discredit him.
Rather than being angry that day, the protest was a "fun thing" for him, and a "bit of a giggle", he told the court under cross-examination.
After the court had heard evidence from eight witnesses, four from each side, Hume's counsel Stephanie Burlace argued that it was a case of credibility and that police prosecutor Sergeant Rachel Willemsen had not proven beyond reasonable doubt what happened.
There was clear motivation on both sides as to what was alleged and what transpired, she said.
Ms Willemsen argued that there was no motive for the complainant to fabricate her complaint and it had been distressing for her. Hume had been aggrieved enough on the day in question to call the media.
In making his decision, Judge Matheson said he noted in particular the evidence of a former employee of the same company, who was in the office the day the call came in.
He gave balanced evidence that was not necessarily complimentary to the complainant.
He gave evidence that he saw the complainant pick up the call, and within 10-11 seconds be demolished to hysteria and tears, indicating that it must have been something significant.
The defence had tried to paint the complainant as a hardened proprietor who was unpleasant in dealing with staff and, while he made no decision on that, it seemed that if that were true it made the observations of that witness even more pertinent.
Her reaction as described by the witness was consistent with someone who had heard what the complainant alleged they heard, Judge Matheson said.
What happened did not signify a "set-up", no one knew about the protest except the media, and it rather identified that Hume was agitated and in a state where he was prepared to engage his children in a public protest.
That, coupled with witness evidence and officers' observations of him being a blunt personality, and "initially agitated" on other occasions they had dealt with him, helped him come to the guilty verdict.
Hume was remanded on bail to reappear on June 2.
