Each kind of dialogue has different formatting in screenwriting that you have to familiarize yourself with. This is done so that people are able to identify the difference right off the bat without having to read too closely.
In your screenplay, you are going to have a number of different situations where the characters are talking. And, all of those that are not the standard in person conversation that you are going to have a unique dynamic. One of the standard alternatives that get used in screenwriting is the telephone conversation, and there are a few alterations you may to the standard dialogue that you write for telephone talking.
There are two essential ways that you can do a phone conversation. You can do it so you see the primary character, and you only hear the modified voice of the person on the other side of the phone, or you cut back and forth to each character, giving them equal importance in the conversation. The first kind should only be used for shorter conversations where the other person is not near as critical to the specific situation.
If you want to do the back and forth, you have a few choices. You are going to use the consistent screenplay direction INTERCUT to show that you are going back and forth. If you want to just indicate that you are going to be cutting back and forth right at the beginning of the scene, you can just create an INTERCUT type scene by including this in the heading. Example:
INTERCUT – PETER’S OFFICE/DENISE’S BEDROOM
This will show that the scene is taking place in both locations. If you do this, then the director, producer, and editor have the choice as to when they want to cut back and forth. If you want to keep the power when screenwriting, you can just put the INTERCUT in between each line of dialogue. If you do this, you essentially put the INTERCUT and the location above each line.
If you just want to have one character talking on the phone without cutting back to the other one, you simply set it up like a regular scene. If you are not going to hear the other person, then you can just include direction in between each of the single person’s dialogue, indicating that they are listening to the other speaker.
If you want to listen to the other person on the line, then you just format it as a regular conversation, except that you indicate at the beginning of the scene that they are talking on the phone, and you can still hear the other character.
At the beginning of each phone scene, just indicate that it is a phone scene. This can be done by simply adding PHONE CONVERSATION at the beginning header so that the reader will understand this immediately.
If you are only watching a single character, but hear both, you may want to indicate that there will have to be a phone alteration done on the secondary characters voice to maintain the phone sound. Either way, as long as you follow the standard dialogue formats with indications for phone calls and the way you want the scene to look, you will be fine in your effort to communicate.