We just had someone call in with the complaint that there was static on 1 line of their phone system. Upon further investigation it turned out the issue was 1 particular telephone causing noisy static when it wasn't in use. Apparently 2 different Bell technicians were on site for a significant amount of time and could not figure it out.

I couldn't quite remember the code off the top of my head, but Feature # 86 solved the issue immediately!

Ha!
Static on the line. While on hook!
They would have freaked out if there suddenly would have been voices. Or music.

lol, I have had a number of those calls in my years.

Typically if a set was allowed to use F86 in the first place then there must have been a music source attached and Background Music turned on.
I would call the client and see if you get silence when on hold, then either check why the music stopped working and/or change On Hold to Tones and Background Music Off.

Silence is...silent.