Scam debt collector, attempting to gain further identity information. Do not answer! If you take their call they will harass you for weeks, in clear contravention of the Telecommunications Act.
The Legislation: Section 43 Telecommunications Act 1984

(1) A person who –
(a) sends, by means of a public telecommunication system, a message or other matter that is grossly offensive or of an indecent, obscene or menacing character; or
(b) sends by those means, for the purpose of causing annoyance, inconvenience or needless anxiety to another, a message that he knows to be false or persistently makes use for that purpose of a public telecommunication system,
shall be guilty of an offence and liable on summary conviction to imprisonment for a term not exceeding six months or a fine not exceeding level 5 on the standard scale or both.

debt collector

keeps calling no one on the other side.

Debt

You missed a call from 0401 552 944, who left a message that could not be converted to text. This message was provided by Telstra at no charge to you.

You missed a call from 0401 552 944, who did not leave a message. This message was provided by Telstra at no charge to you.