These phones used to use mechanical switches for selecting which line was being used, and then they often had a 'Conference" button that tied two lines together. Life expectancy of the "Conference" button was about 3 weeks, and would stick, tying up several lines. Best way to test is unplug ALL the phones, then start plugging them in, testing till you find a problem.

Sometimes the thoughts in my head get so bored, they go for a stroll through my mouth. This is rarely a good thing.