I'm pulling this out of my ass, but...
Police have to get the plate number. Probably from the owner, since they might have a make, partial plate, and color to go on. Volts look unique and all, but not in a "That's a VOLT!" way to the general public, more "Hey, what model is that? It's pretty cool" way. So, find owner if that's not who reported the theft in the first place.
Next, they need the VIN, which they get by looking up the registration from the plate. Need to write that down because not sure what part of VIN is important.
Find and call OnStar's outside number. (Do you know what it is? I don't. I'd have to look it up.)
Wait in whatever hold queue exists for people not calling from their cars already.
Inform OnStar that car is involved in an active police chase and needs disabling. Give PD jurisdiction. Give VIN. OnStar rep initiates check call to PD dispatcher to confirm that this is real, and looks up car by VIN. If OnStar Service is not active, starts activation process using override codes.
Once dispatcher confirms that call from officer is legit and account comes active, OnStar rep ask for confirmation that it's okay to run the disablement so that OnStar is not liable for any problems the disabling will cause, pushes button.
OnStar system initiates contact with car and a couple dozen seconds later, car goes into disable mode, however that's executed. (Personally, I like the idea that it goes all horn honking and lights flashing and stops being able to be accelerated faster than about 15 MPH, but I don't actually know what happens.)
If cops haven't already surrounded vehicle by then, thief flees on foot. Once cops have vehicle secured, this gets relayed to cop on phone with OnStar who tells them they can turn off the disable.
That's pretty fast, but I can easily see the whole process taking 15-20 minutes IF someone's thinking about it. There's probably shortcuts: EG if owner IS onstar subscriber to the right level of service, then the number to call is in the app. The account number to identify the vehicle is in the app. The owner will know the security PIN to authorize the security stop, and the car will already have an active account, so there's no processing there. That'll cut the whole deal down to a couple of minutes, but that kind of depends on having all of that kind of stuff set up in the first place.
