By default its the ext Bell - terminal but it is programmable as anything so if changed won't work as a bell
But it 'could' be a broken core or knackered bell
its also standby time. You can calculate the approx demand plus switched mode power supplies dont deliver over their rating and the texecom ones have issues anyway
It's highly unlikely its been well maintained, few people want any sort of manual for anything that is. However assuming it hasn't been well maintained I'd go with Mr h's findings