Craigslist is fighting back. Its latest gimmick is phone verification. Posting in some categories now requires a callback phone call, with a password sent to the user either by voice or as an SMS message. [...] Spammers tried using their own free ringtone sites to get many users to accept the Craigslist verification call, then type in the password from the voice message. Craigslist hasn’t countered that trick yet.