It is most surely an unsafe caller.  I don't answer unknown callers either, but it follows the pattern set by other ne'er-do-wells that have called before.  They call several times a day, usually at inconvenient times - early, late, etc, only ring 5 times to avoid the voicemail picking up and never leave a message.  It is apparently some type of scam that requires actually talking to the "mark" as opposed to leaving a threatening message requiring you to return the call.  Of course, you can't return the call because the number you see, if you see one, doesn't actually exist.  It's been "spoofed".  I've even gotten several calls that showed MY OWN caller ID information.  Take heart, though, the prison cell that awaits these criminals is equipped with a phone that gives 5 rings once every minute 24 hours a day, 7 days a week and cannot be answered.

We have Vonage, Ooma and other VOIP operations to thank for this glorious new way to fleece the population anonymously.  It's too bad that these same brains can't come up with a way to trace the calls and put a dent in their operations.  The sad part is that they must be making a profit or they wouldn't keep at it.  Their victims are usually the ones who can least afford to lose the money.