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.
The hotel case you mentioned doesn't seem to involve that; in this case, the hotel is opposing the request, so there's an adversarial posture between the third-party data holder and the government over handing over the data.