You're stuck on the color when it has no particular relevance that changes the nature of the question. I just happened to select white because it was one of many colors commonly referred to when the practice was in widespread use. There were other references also, such as "house phone" or "lobby phone", "paging line", etc. ~
Not white. I thought it meant something too, but after much debate it seems that it was just a courtesy phone. Being white wasn't pat of the reason it was courteous. Had me fooled!! :)