I don't know who it is but they keep calling my mobile and not leaving a message and I am sick to death of it

I just had the exact same thing happen to me, tried to call it back and it's just engaged.

It's Telstra

Same thing happens too me, miss lots of calls from the same prefix 038664...., when i answer no one there, cant call back??? weird

Called Telstra's "nuisance calls" centre on 1800 805 996. This number is known to them: it's a telemarketer.They use a computer to dial numbers at random, and it records only whether the number is answered or not. If so, it becomes "more interesting" to the automated system. When a number is often enough answered, it becomes interesting enough to be allocated to a human telemarketer.

it's telstra