CrimsonKidCK:
AFAIC it's both challenging and fun, attempting to imply what the 'unheard' individual is saying via the other one's conversation with him/her...
This is definitely true, and it adds a level of complexity to the story.
I used another approach in the story
Cell Phone where I placed one end of the conversation in italics, reasoning that sometimes people with good hearing can pick up the other end and to suggest the fuzziness of the telephone voice. Then, in case that wasn't believeable, and because of the nature of the story I did this:
"Hello, Mark Williams."
"Hello Mark, It's Jenny.""Hello Jenny. Do you want to speak to Mary?"
"No, it's both of you actually.""Just a mo, I'll put you on loudspeaker."
"Look, it's about the handbag, Mary."...