Try discussing that with the family of the murder victim Sally Geeson. She wrongly believed an emergency 999 (911 to North Americans) cost money so she did not call when she knew her life was in imminent danger as she had no credit left on her phone:

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."