While there's no standard, there are several conventions I have seen.
For formal transcription, as noted in the comments, you'll often see brackets (or asterisks used as brackets) with a descriptor.
"John [mumbled] party next [static] week. I told [repetitive clicking] she could [line cut]."
For fiction and other informal writing, often the writer will describe the type of problem and then use some symbol to denote where it occurs or use an onomatopoeia for flavor.
Ex: The line was terrible, I could barely hear over the static. "John @&*% party *** week. I told … she could KWERSHHHHHHHH" and then the line cut out.
I've mixed several styles above, you should pick one and stick with it. You can also use the transcript method in fiction as well, if appropriate to the genre.