One approach is to lay out the dialogue that can be heard and use narrative to help create the pauses that occur when that character is listening:

“I’m not sure.” My mother sat at the kitchen table. She held the phone to her ear with her shoulder and bit at the cuticle on her thumb. “Quentin wouldn’t lie,” she said, standing up to pace the narrow kitchen. “He’s not that kind of boy."

This approach is particularly useful in a circumstance where the words spoken during the conversation are important. Quentin might be eavesdropping to find out just how much trouble he’ll be in when his mother gets off the phone. There may, however, be situations where the exact dialogue of the exchange isn’t as important as the mood of the moment. In such cases, you might focus more on action:

I turned out the light on my night table and still, Linda chatted, the cordless jammed to her ear. She laughed that high-pitched giggle she uses only when she’s around other women. I put my pillow over my head.

The emphasis here is on the narrator’s annoyance at the noise Linda is making when he’s trying to sleep, and at the act he feels Linda is putting on for her listener. What Linda says isn’t nearly as important as how she says it, or how the narrator reacts to it.