Lately, this is the type of call I've been getting:  not recognizing the number, I let the call go to my answering machine.  No message is left.  I use *67 to mask my own Caller ID and call the number back.  I get the telephone company recording saying "this number is no longer in service."  All of the calls like this that I've received have the same area code and prefix, so it's probably from a boiler room operation with all the originating numbers forged.