I found this article regarding telephone calls to the girls that advertised on Craigslist. According to this article calls made to Melissa's mother and sister were made from a pay phone and a disposable phone. Also, calls made to the escorts from the Craigslist ads were all less then 3 minutes and made on disposable phones. That may be why numerous calls were made to the girls before the actual dates. He was limiting his calls to under 3 minutes knowing the calls could not be traced.
"The several phone calls remind detectives of an earlier clue: the client of another girl who ended up on Gilgo Beach, Melissa Barthelemy (above right), made several calls to the home where she lived with her mother and sister, even taunting them after she disappeared. Police traced those calls as coming from Times Square, Madison Square Garden, above Penn Station, and the Long Island suburbs near Babylon.
Whoever made those calls knew how to avoid detection: they were from public booths or disposable mobile phones in crowded places, and each call lasted less than the three minutes it takes a cop listening in to trace them. All the calls responding to the Craigslist ads came from a disposable mobile, and lasted less than three minutes."
Read more:
http://www.theweek.co.uk/politics/6133/craigslist-killer-growing-sense-he%E2%80%99s-cop#ixzz22899JTsC