Apparently spam number according to internet comments - three calls in 2 days.

cold call trying to get me to change power providers

spam

Someone or something from this number called me, nothing was said they just/it hung up. I called the number only to get a message from Optus advising me that the number was disconnected.???

powersave company

Called the number back only to receive a recorded message advising that the number dialed was incomplete........
From this, I can only assume it's a telemarketer.