As others have mentioned, the scammers simply run through a sequence of all possible phone numbers. The scammers system/computer readily identifies which ones have a service, especially if you answer. Some numbers return a telco re index message to advise that the number does not have a service.
The most one can do is go only answer calls your mobile recognises. Telstra offer a home phone/service (Call Guardian) that only passes on calls from a list of numbers you set. All others can go to a message bank, at your discretion. The phone answers the call via a recording, enabling the home owner to selectively screen or ignore callers.
