If you’ve ever had to be in the same room as someone on the phone, you’ve also had a huge chance to listen to half a conversation. The half you hear is just as important as the unheard half.
As an exercise: Take a two-person conversation from a story (yours or someone else’s). Designate Character A and Character B.
Now, remove all of Character A’s lines. Using B’s half of the conversation, come up with two or three possible responses to each line of remaining dialogue. How does the conversation change when you only have one half to infer information from? How might changing this conversation change the story?