Actually Mel, I think you are doing them a disservice by telling them that they don't have what it takes. And anyway, what does it take? Certainly not a great voice. Or great acting skills. Being an actor is not like being an NBA or NFL player. Acting is a skill that anyone can learn, if they have the time, patience and desire.
I've found that desire and determination are at least as important as talent. In fact, I've often found that laziness has cursed many of the talented folks I've come across. Whereas those of us with less raw talent work much harder to make up for our lack of skills. And I maintain that no-one can tell how determined someone is from a phone message