They could also find one of the few payphones (I know where some are) and use them. Or business courtesy phones. BFD. the idea that all cellphone calls should be tracable to the caller is psycho police state paranoia and beneath the American ideal.
if you are prepared to grant a new civil right of anonymous communications capability - be preapred for more terrorism. communications intercepts are a vital tool in proactive anti-terror methods, and if you can't trace the call to the person making it - the intercept itself is of little value.