OnStar uses blended hybrid technology to report your location and to allow you to place calls. The on-board "module" contains three primary components: Cell transceiver, gps receiver, and an Airbiquity unit. When you press your button for assistance, a call is placed to the OnStar call center. This call takes place over whichever cellular company is providing service at that particular location(VZW, AT&T, Sprint, etc.). While you're on the call, the Airbiquity unit is the primary component responsible for relaying gps and diagnostic information to the OnStar call center. Airbiquity patented this technology to allow a data stream(gps coordinates, diags) to traverse a voice path while you are on the call. It is overlaid. Pretty cool stuff. Otherwise without this tech, the voice and data streams would have to be sent over two separate paths, or two different "calls." Calls are not made to a satellite(s). You must be in range of a cellular network to be able to place a call or have your vehicle automatically send out a crash notification. For the most part, almost the entire freeway system in the US is covered. It's once you get out in the boonies where there is no coverage is when you're screwed.
Jourles, whose company just shut down their old analog service last week which serviced the old OnStar modules