Math Based Magic Trick

Here is an amazing math feat YOU can perform.  Since this is February, we have added a little Valentines Theme.  Check out this lightning addition trick.  

Not many tricks can be done over the phone.  This is one! You and your friend alternate turns, each selecting a number from 1-9.  The object of the game is to be the first to have 3 of your numbers total 15.  You have a secret strategy that lets you win much of the time.