If there is silence then you get awkward conversation usually about Uber. One day someone told me to turn on smooth jazz. So after flipping through Sirus XM I came upon channel 66 and 67.
Keep the music low and in the background. On my Toyota thats music level 4 or 5. If the passenger gets on the phone immediately turn it down. When you get on the highway turn it up, off the highway turn it down.
Even if they dont like jazz I find its better than awkward silence or awkward conversation. Keeps everyone focused.