This is an Optus call centre number

I've gotten a few calls from this number, by chance I was never around to answer it, but I was today and when I picked up there was a guy on the other end said hello?, then as soon as I said something, he hung up

This number is driving me stupid - regularly calling and then leaving no message.

Very glad to hear it's optus, but they call every few days at around 9 am. and never leave a message.

Same as above. Glad to hear it's an Optus call centre, not a stalker.

Yes - calls me probably every 2nd day. Leaves no message. If I answer no one talks, then they hang up? Been happening to me for 3 weeks now?

Calls daily and does not leave message