You might be right, but actually I am tying up a line that they need to run their scam, so numbers that do not initiate a conversation does have a cost in terms of efficiency. 

The fact that they are programmed to hang up after 8 seconds of silence indicates to me that they are trying to be efficient with their resources. Why would they call that inactive number again when they hung up on it to begin with? That's all I'm saying.