He was forced by Monis to make the call to police at around 9.41am.
"I'm calling from Martin Place. I have a message to read to you," Johnson begins.
Despite being frequently interrupted by the operator during the 12-minute call, Johnson remains calm.
"There is more message that I have to read to you as well," he continues.
Operator: "All right, well you just need to hang on a second."
Johnson: "I need to finish reading this ..."
Operator: "Yes, I understand that. Just hang on a minute."
Johnson: "I have a gun in front of me."
Operator: "OK".
Johnson was later able to read the note in full. The note, which was shown to the hearing, reads: "Australia is under attack by Islamic State. There are three bombs in three different locations: Martin Place, Circular Quay and George Street.
"I want to contact other brothers and ask them not to explode the other two bombs but I can't contact because they don't carry phones with them. They have radio with them. I can say that through radio ABC. The plan is to request Tony Abbott to call them or me and to have a debate while it is broadcast live on ABC national radio.
"So that is why they have radio and the best way to contact them is a voice message which will announce that they should not explode the bombs.
"They listen to anything I tell them. The device placed inside the radio is another way of exploding the bombs."
Counsel assisting, Jeremy Gormly SC, told the hearing that Johnson had demonstrated "calmness" and "command" in the situation.
The hearing was also told it was possible Monis had planned his attack for months, having acquired a mobile phone registered in another person's name on October 15, 2014. He did not use the phone until the day of the siege.
"That could suggest that he had the siege in mind as far back as October, 2014," Gormly said.
It was also possible he had hoped to escape, Gormly said.
"The fact that Monis had a different mobile and was seemingly protective of his identity raised the possibility that he had some intent or hope of escape and evasion from police."
The inquest continues.