Scammers don't let the phone ring more than 3 times, they don't want their number logged on your caller Id. If you set your answering machine to answer on the first ring, you will spook them into hanging up quickly. Legitimate callers will leave a message.
Got a call from 325-307-5508. I don't answer calls where I don't recognize the number/area code. Called at approx. 8:10 on a Wednesday evening. No message left.