I called back number (03) 9427 4406 reached IVR for a company called Telstra Business Center.

The 'Truecaller' app says it 'Telstra'
timewasters
block em

Caller: TSAGROUP-100% Australian-owned business process outsourcing company specialising in helping companies acquire, retain and grow their consumer and business customer bases. eight call centres in Australia, and two call centres in the Philippines.

Rang & spoke to someone who could access Telstra account re nbn connection. These calls advise progress of account management prior to receiving nbn order number. So in my experience I recommend you get back to them. The reason they do not leave a message is due to their privacy policy.

I keep getting calls to my mobile and they leave no message on my voicemail. So i have blocked number

Phone number rings often and then hangs up

This number constantly calls my mobile and hangs up. I have now blocked it!

Often receive a call to my mobile which hangs up on me when I answer. This will be followed shortly by a call to the office phone and when myself or staff answer the phone, the caller hangs up.

This is harassment and to who ever this phone number belongs their intent is certainly not to be professional.
I suspect it is a call centre with idiot staff

Origin Energy again possibly?