I called back number (02) 8123 4057 reached voice mail but did not say for what company.

Don't call me

I have had 12 calls from this number in the last 2 weeks. When I answer I get a busy signal, probably a computer. I've called back to ask to be removed from their calling list but a voice message says we will call you back. I've blocked the number but they call back with the same 8123 prefix, but different combinations of the last 4. How can I find out what company this is and how do I report them to stop getting their menacing calls? With phone scams so advanced these days, I'm concerned about the frequency and reason for calling.