There are some great tips for writing dialog that the reader only hears one side of in this article on Writing Phone Conversations to Forward Your Plot and Draw Your Characters from the BookBaby Blog, including this:
After you have written the first draft of your phone conversation, read it aloud and ask yourself the following questions:With suggested exercises and even an invitation to have your dialog critiqued in the comments section, it's a piece that's well-worth reading.
Is it subtle enough, or am I hitting readers over the head with information?
Does it sound true to life?
Am I communicating to the reader what he or she needs to know, and nothing extra that has no purpose?
Is the conversation moving the plot forward?
Does the conversation give insight into the character of one or both participants in the dialogue, or the relationship between them? Or does it enlighten the reader about another character not participating in the conversation?
Are emotions and motivations being conveyed?
Illustrate and Write On,