Charge the call, if the call is justified give a discount of 100%.
From http://www.theguardian.com/technology/2006/oct/19/guardianwe...
"...tragic consequences last year when Sally Geeson, a student, was abducted and murdered in Cambridge. Sally, whose phone had run out of call credit, sent a series of texts asking for help to friends, apparently not realising that she could have made a 999 call for free."
I really believe that this would be a more efficient system on the whole.