Someone above mentioned picking up the phone and then hanging up before they scammers could get started with their automated sales pitch. Do NOT do this - just by lifting the phone (or hitting the green/on button) to answer - even if you hang up immediately - you are only CONFIRMING for them that your telephone number is valid and even if they can't sell you on their scam this time, now they know to keep dialing your number. And they know your number is a valid one, so they can also sell it as a part of their database of verified working numbers to *other scammer*.
Any number that you do NOT recognize you should NOT answer. If it is a legitimate caller, they will leave a message on your voice mail or answering machine, which you can choose to return (or not) at your convenience.
There are even companies calling _only_ to see *IF* and *WHEN* people do answer. They are amassing a database to sell to other scammer companies of not only what numbers to call, but the best time(s) of day and best day(s) of the week to call, o as to catch someone at home who will pick up.
That depends .. on my phone lines, picking up on the first ring alone can toss it into oblivion just by picking it up too quick. It will disconnect the call just that quick. But we are talking about phone lines and equipment that is of BC age.. Allowing these calls to go to a answering machine just verifys they have a live number also.