The convenience part I can understand, especially with the demise of common pay phones.
Just out of curiosity for discussion purposes, how does it offer "safety"?
That is an argument I hear very often as justification for giving a good child a phone (and indeed why I originally got one myself).
For myself, I got one primarily in the case my car breaks down. If your daughter is driving, then that is a reason, especially nowadays with few pay phones out there. (However, in ten years I have yet to use the cellphone for that purpose.)
If the child isn't yet driving, I wonder about the safety issue. In other words, when I was a kid that was never an issue. My parents' biggest worry about me outside the home was traffic safety, being hit by a car carelessly crossing a major street. I think that was the leading cause of accidents for kids in those days. When I was older and going further via the subway, they were nervous about muggings on the subway (with good justification). I did not run around late at night.
Anyway, I wonder if today's pre-driving kids are exposed to so many dangers out there? I can't help but wonder if the biggest danger is unsupervised younger kids looking for something to do and peer group pressure, such as pushing a hesitant 14 y/o to go to the woods or an empty house to smoke or drink. In my day parents were home and are whereabouts, esp pre-16, were pretty closely monitored and structured without cell phones. We wouldn't allowed to go that far from home to begin with.