Simply put, this will be a listing of things it's better to not have in your script and why. Understanding that there will always be someone that can point out that one or two films have had this and have still been successful, the majority of successful films avoid these traps.

- Extended one sided phone calls.
They are boring and little more than exposition. In classic drama, the hero would either summon or be approached by a messenger bearing new information for the audience (think Romeo and Juliet when Romeo's servant Balthasar arrives and tells Romeo that Juliet is dead). The classic line "Ho, what news from afar" is the idea. Same thing with the modern telephone, it is simply news from afar. Keep it short and sweet and move forward with the story. Having an extended phone conversation where the audience only hears one side of the story means the hero has to commit the dreaded act of repeating everything he is hearing or else be privy to information that the audience doesn't know, only to turn around and tell another character what he just heard.

Having the voice heard on the other end isn't hard, production wise, by the way and it brings the audience into the world of the story.

In addition to this, try to have your hero doing something while making the phone call. Much better than just sitting on the phone. It means that the hero has to focus and is in risk of danger, always good in a script. Car chase, looking for evidence, anything that requires concentration means that the act of talking on the phone is much more than just a casual conversation talking on the phone.

There are always exceptions to the rules....
cheers
Q