Lim Bock Song, 43, blurted out what he had done.
But before the shocked woman could ask him any questions, he hung up.
The call stunned Lim's 60-year-old mother so much that it took her more than an hour to compose herself and call her youngest son back.
But there was no answer, for he was dead by then.
A police officer had shot him in the chest on the North-East Line platform of OutramMRT Station at 3.25pm on Thursday.
According to the police, Lim, who was unemployed, had charged aggressively at the police officer with a knife when he was approached by the latter and another police officer. The shot was fired at close range and Lim was pronounced dead at 3.50pm.
Lim was believed to have stabbed odd-job worker Tan Ah Chang, 52, at the Jalan Kukoh hawker centre at 2.45pm that day.
Yesterday, the families of both men were at the mortuary to identify their bodies.
Both families declined to be interviewed, but a family friend of Lim's later told The New Paper that his family was still trying to come to terms with his death.
He said: 'His mother was watching the evening news and that was when she realised her son was dead. She recognised the shirt he was wearing.'
He said the police couldn't contact Lim's family earlier as he did not live with them.
He added that Lim had been treated at the Institute of Mental Health (IMH) for five years. In May last year, he moved in with two friends to a one-room Jalan Kukoh flat.
The family friend said Lim was 'depressed' after his divorce. It happened when his son, now in Secondary 1, was in kindergarten.
The friend said: 'Bock Song thought too much about his son. Recently, he tried to get access to him. But during Chinese New Year, his son called and said he was doing fine without his father.
'I've known him (Lim) for many years and I think he was very affected by that.'
The family friend said Lim had been on medication for depression.
He said Lim's mother used to call her son regularly as she 'still loved him', but the other members of the family were not in contact with him.
It is believed that Lim had stabbed Mr Tan over a money dispute.
A drinking buddy of Mr Tan said Mr Tan and Lim did not get along well, Lianhe Wanbao reported.
He said: 'I heard that their (problems) started because Lim had borrowed some money from Mr Tan's friend.
'He did not return the money and Mr Tan was helping his friend to get it back. But he got on Lim's nerves and was killed.'
Lim's neighbour, who wanted to be known only as Martin, said that Lim's family owns a business and is well-off.
Mr Martin, 62, a security guard, said: 'He was different from the rest of us. We had no money to buy beer and eat outside. But he (Lim) was always dressed smartly. He used to go for a drink at 11am every day.'
The family friend said Lim used to help out at his father's coffeeshop until the elder Mr Lim died 'years ago'.
Another neighbour, a 72-year-old man, said Lim used to work as a cleaner at the same hawker centre where the stabbing took place.
He said: 'I asked him why he stopped working. He said it was because his hands kept shaking and he would drop plates and bowls when he cleared the tables.'
The neighbour said that he last saw Lim a few days ago. Lim had greeted him with Chinese New Year wishes repeatedly.
He said: 'He seemed to have mental problems, but he looked quite refined and had no tattoos or anything like that.'
At the hawker centre yesterday, a close friend of Mr Tan's pointed out his backpack next to the table where he had been stabbed.
The bag contained work tools as Mr Tan had gone for a drink after being given the day off from work when it started raining.
His friend said: 'If he hadn't had the day off, he would still be alive now.'