At same positioned game objects collision detection

In my game, I am creating two game objects at same position using Instantiate statement. Each generated object has box collider attached.

So I want to detect collision between them so I write following code.

 void OnCollisionStay(Collision collisionInfo){
         print("on collision stay");
     }

But this statement not called. I generate two object using following code.

 BallUtility.AddNormalCommonBall (normalBallPref, Vector3.zero);
 BallUtility.AddNormalCommonBall (normalBallPref, Vector3.zero);

I need help in collision detection.