This is from an informative article on the Telstra website. (I've edited it slightly.)

What is call spoofing? How scammers are using your phone number for fake calls

Fake calls are on the rise, and the perpetrators are using old tricks in new ways. It’s called “call spoofing”, and it’s where real phone numbers – sometimes even yours – are being used to scam others. Here’s how it works, and how to stay safe.

First things first: Telstra is spending a lot of time and effort developing and progressively rolling out system capabilities that can detect and block scam calls before they reach their customers.

These systems aim to block calls originated from off network sources that have suspicious calling patterns or that spoof or use unassigned numbers to gain credibility when dialling their victims on the Telstra network.

If a Telstra customer has been the victim of a call spoofing scam, they can report it to Telstra via the Telstra website.

What is call spoofing?

Call spoofing is where a phone scammer will falsify the calling number to trick a user into picking up or making a call look more legitimate.

It’s a global problem, and it’s unfortunately on the rise over the last few years.

Spoofing occurs when a scammer originates calls, usually via “robocalling” technology, with fake caller IDs i.e. the scammers are using technology to originate calls with legitimate Australian numbers included as the caller ID.

Telstra has strict controls that helps prevent the possibility of caller ID spoofing from arising internally within its own network. However spoofing can still occur with calls that reach Telstra customers when originating from outside of the Telstra network.

Scammers know that people are more likely to pick up the phone to a number that looks legitimate, rather than one that appears as private, blocked, or from overseas.

Scammers may also call you from spoofed numbers that are “adjacent” to yours. It’s a social engineering tactic designed to increase the chances you’ll pick up the phone. It’s a new type of spoofing that uses your so-called ‘number neighbours’ against you.

For example, if your number is 0400 000 000, they may call you from 0400 000 001 to increase the chances you’ll pick up.

And some people have called by what looks their own number. If you have experienced this, the scammer has spoofed your number and by chance has called you as a potential victim, so that it looks like you are calling yourself.

Rest assured Telstra is working on solutions to prevent as much spoofing as possible.

What happens if you pick up a spoofed call?

Once the target picks up the phone, an automated message will play, often demanding money and threatening fake consequences if there’s a failure to pay. Telstra has noticed phone scammers recently impersonating government departments like Services Australia or Border Force; Amazon; eBay and sometimes even Telstra or NBN Co.

Some call spoofing attacks even connect you to a real person whose job it is to extract cash from you under false pretences.

Whether personalised or automated, the scammers often apply time pressure and the threat of sanctions or penalties if you fail to pay while on the call.

They may also be after other identifying information to use in other fraudulent activities, so it’s important you don’t give out any of your personal information while on a call and if you suspect it is a scam call, hang up immediately.

Call spoofing may use real people’s phone numbers to execute an attack. This can lead to difficult conversations when the victim calls or texts the number back and reaches a legitimate service whose owner has no idea their number is being used in a scam ring.

Here is how you can spot scam calls when they come in.

How can I tell if my number is being used for call spoofing?

If your number has been spoofed by a scammer, you’ll often find out right away.

If a victim were to redial the number they were scammed from, they would reach the rightful owner.

If that owner is you, a barrage of unexpected abuse might come your way . Someone might even just call you out of the blue to ask why you tried to scam them or demand to know why you’re harassing them.

That’s a good indicator your number has been used in a spoofing scam.

Telstra say that they are ❝actively blocking scam calls at a network level, meaning that these sorts of calls won’t be able to reach your device. On average, we’re now blocking around 13 million scam calls per month.❞