Australia Reverse Phone Lookup

constantly calling our switchboard - yesterday someone actually asked to speak with ?? & before i could say anything they hung on me .. and now they are calling & hanging up at least 3-4 times daily. .. how can you stop this from happending ?

Received unsolicited call allegedly from "xxx Energy" (forgotten name, but from what I've since read, they use different names) offering an 11% electricity discount. I asked "Who is xxx Energy?" The full response: "An electricity company". I polite replied "Yes, but can't you tell me anything more?", to which they hung up.
Out of curiosity I called back. A recorded message did not identify the number, but instead advised a lengthy wait before the call would be answered, immediately followed by a request that I leave my name and phone number "after the tone", then followed by "the mailbox is full" and disconnection.
Unethical and/or incompetent time wasters.