There are many ways to look at this. lets say you haven't seen a relative for several years and all you can do is use a phone to talk to her/he or you could say maybe in restricted environment like North Korea, you and your mother have been separated ever since the country was cut in half, your mother has finally got a chance to talk to you after five years and the only way is through a phone. You could use a phone for emergency cases like if you stranded or if someone has a heart attack at home and you were the only person there at the moment, you don't know how to drive so you would call for an ambulance lol. I hope this helps, I tried listing a lot of ideas.
Advice: Use your cell, but you ought to consider the value of text-messaging over an actual call. Time Magazine has an article this very week's issue about how good it is to text message. Everyone will read a texted message, whereas an email they may not.