Caller: An annoying someone or something, constantly phones!!

These numbers, all starting with 0324 plus a heap of other numbers (too many to mention!!!) constantly phone then hang up. I've dialed a couple of numbers back and they don't exist, according to the recorded message I receive.
They start ringing from 2.30pm and keep going up until around 8pm. IF I answer the phone, no more calls come through, what the hell is that all about?