Yup, for speech recognition the optimal environment is always as quiet as possible, since background noise will either occlude the speech or cause an attempt to recognize it. So if the users are in the car and they are just using the built-in phone mic, it’s a good suggestion for them to turn off the radio. The important thing about calibration is that it is done on an environment that matches the speech environment, meaning that if the user is going to talk over the radio even if you suggest that they not do that, you want the radio on during calibration because silence in that case means “the user isn’t talking but there is quieter radio noise running in the background”.