If it's something severe, they'll call you on the day they get them and force you to come in.
They don't give results out over the phone because they're uncertain who is on the other end. What's to stop me from phoning up, putting on a fake voice, and getting your results?
It's probably all clear, but it could be any deal of minor problems, like dehydration (me) , slightly anaemic (alot of girls in puberty), low levels of certain vitamins etc.