The factory Bluetooth for calling is in the On-Star module between the radio head unit and main glove box. Because of the way Bluetooth operates you should have good reception anywhere in the passenger compartment as well as up to 30 ft (10 meters) away from the car as long as the ignition switch is on. For those with the Bluetooth PDIM or built in Bluetooth stereo, this antenna is below the radio (PDIM) or in the head unit itself (2013 & later). Again, anywhere in the passenger compartment or up to 30 ft (10 m) away with the ignition switch on.
I know as soon as I get in my car with my Galaxy S4 and turn on the ignition, the little "B" will pop up on the display. It doesn't really matter where I am in the car or where the phone is. Just make sure it is configured to pick up the Bluetooth along with your phone and they are "Paired".