The best I've found
was back in the days when electro-mechanical PABXs were the standard - in this specific case, what was known as a BPO* 100-type.
My off-sider and I went into the PABX room at a small business and found the manager's outboard motor leaning against a wall, complete with a partially full tank and a general aroma of petrol. Cue very prompt removal of motor and petrol followed by ventilation of the room.**
We prevented this from happening again by removing the all external and internal covers from the PABX (especially and particularly the 'pulse set') , taking the manager into the room and letting him watch calls being made - with the lights out, so he could see the sparks more easily.
* British Post Office, for all you young'uns.
** This was many years ago - nowdays we'd have to have evacuated half the block and called the Fire Service, Police and $deity knows who else to deal with it, as well as filing sundry H&S reports.