This is an expression we hear very often from reporters during elections and from commentators during one-day matches. When you say that a game is too close to call, what you mean is that it is evenly poised, it is almost impossible to predict which team will emerge the winner. When used during elections, what it implies is that the two candidates are running neck and neck and therefore predicting the ultimate winner is not possible right now.
*The counting is going on. The result is too close to call.