As far as I am aware, the OBTP within Spark/Teams would presently only be for cloud registered endpoints, which would imply that they've been scheduled somehow.
You can use the Calendar Connector on the Expressway - C, which would let you schedule meetings with the
@Spark / @webex / etc , that should have some interaction with TMS to bring OBTP for those meetings to premise endpoints. We're still setting this up so I'm anxious to see if it works properly or has a bunch of gotchas too.