Cheapest SIM in Aus? Need second number for new business

I want to get a second number for receiving business calls and WhatsApp messages, but the cheapest plans I can find all start at $15 a month. Whilst this may seem cheap for some, I’m literally only wanting the number to be able to receive messages via a secondary WhatsApp account for my business, so that I don’t have to give out my personal number to strangers on the internet.

It doesn’t need any data included with the package as the WhatsApp account it’ll be registered to will be signed in on my phone which uses my personal number’s data.

I’ve tried searching but nothing below $15 seems to exist. Does anyone have any recommendations?