I'm not sure, but at one time there were also a lot of party lines. A dial tone was a way of signalling that you had the line to yourself and you weren't pickin

I'm not sure, but at one time there were also a lot of party lines. A dial tone was a way of signalling that you had the line to yourself and you weren't picking up during a lull in someone else's conversation. Today we have the signal bars to show you status. Back then, a light would have added expense and complexity to the phones of the day.