I called back number (02) 8264 4804 reached IVR for a company called HCF My Health Guardian.

I talk to MY Health Guardian every month, if I miss a call they usually leave an electronic voice message saying they will ring back. Which they do

My wife has been associated with My Health Guardian for a number of Years. This morning while we were out we received a call form (02) 8264 4804. I was a bit concerned so I Rang HCF, one of the health insurance companies associated with MHG. I then rang MHG on 1800 244 854 the number given me by HCF. I was informed that (02) 8264 4804 is indeed a legitimate number and I actually spoke to the foreign girl that had previously rung.

Yes it is a scam to see if the number actually exists

DITTO!
calls then an electronic voice says 'goodbye' as soon as you answer and hangs up

We have received several calls from this number on both our mobile phones.
Most times we miss calls (because of our location) and generally return the calls. But from this number there is never a message.
This morning a female with a very strong Asian accent actually asked for me by my name. But she called me Mr.
But I am a lady!
And mumbled that she was from my health guardian.
I was suspicious.
(And yes, we do have calls from My Health guardian every month. And to a genuine staff member of My Health Guardian I would have discussed my health issues in detail)
She then transferred me to another female.
When I asked whom I was speaking to she said, very rudely: "Well, if you do not want to speak to me, goodbye" and hung up on me.
I need to find out how to block callers on our (ancient) mobile phones.

nothing then female electronic voice says "goodbye" and hangs up

Has rung the past few days but not left a message. Electronic voice says"My Health Guardian"

Its HCF "My Health Guardian" service as far as I can make out.

calls then an electronic voice says 'goodbye' as soon as you answer and hangs up

No message