I think I see what you're saying now. So, DSL can "ride" on an analog voice line? That's new to me.
I have DSL service from originally half a decade ago. At that time, it didn't appear there were any options BUT to use the 2nd pair for dedicated DSL. In fact, I asked and they said if I wanted to have 2 separate phone lines in the house, I'd have to get the phone company to run another line into the house because the current one would be using up 1 of the 2 [pair] and the DSL would use up the other.