iPhone users that wish to use their factory Bluetooth for hands free calling. In order to retain the use of the factory Bluetooth on an iPhone device you will need to "forget this device" for both the car and the GROM unit if previously paired.
Then connect to the car's factory Bluetooth hands free function.
And then connect to the GROM unit for Music streaming.
iPhones do not have the ability to manually select what function of a device you want to pair to it will automatically pair to all Bluetooth functions that are available on a device, for this reason you will need to pair to the vehicles Bluetooth first then the iPhone will only pair to a GROM units music streaming (A2DP) function as it knows that the hands free(HFP) function is already being utilized by another device.
Note that you will need to do it just ONCE, the first time you set up your phone and GROM.