It's true that in Smith v. Maryland "[t]he police did not get a warrant or court order before having the pen register installed", so the original case was a voluntary disclosure in that sense. But installing pen registers isn't voluntary at all these days:

https://en.wikipedia.org/wiki/Pen_register#Pen_Register_Act

So it's reasonable to speak of the third party doctrine being applied to compelled data disclosures, not just voluntary ones.