This is not a proximity jammer - jamming devices require FCC approval and TRANSMIT jamming frequencies. This cell would operate as any normal cell would, excep that it would NOT allow any outgoing cell calls or data connections while the car is moving. IT could for that matter, learn and store the cell ID from the flash card in the cell phone, so that after a few repeated connections to it, it would learn the numbers to be censored. Depending on the design of the micro cell, you couldn't "unlink" the phone, regardless of whose cellular plan you were on.
It is the cell controller itself, not your phone, that determines whether it will "allow" your device to connect to it. For that matter, it could still allow "911" calls. Even the presently available active cell phone jammers used in prisons and some schools allow "911" calls to be made, and allow for specific, protected over rides.
GPS functions could be allowed, though most GPS systems use satellite for primary navigation, and only cell phones fall back to cellular tower data for fine interpolation, often inthe absence of satellite data (skyscraper cities). You could also still have your Onstar functionality, too.
IF the car isn't moving, then you could still make your cell call - for that matter, simply turn off the car, and you'd be free to connect to any other cell controller nearby.