If you would be willing to tune your radio to an empty radio station, leave it there all the while listening to the white noise so you can push a button on your phone if you get a call, I'm sure it can be done.

This guy does it for only $10 ...without any additional software.
>> https://www.jupiterjack.tv/?cid=703040


However, notice he says "instead of hearing music".

You could have a preset on your radio to the best frequency for your area that would require two button pushes if you are listening to another station when a call comes in. One for the phone and one for the radio. But then, it would only work consistently well if you stayed in one area while driving your car.
What I mean is, in the Philadelphia area for instance 95.3 works well until you go just north of the city, then 105.6 needs to be tuned. If you go south then 88.9 is the best frequency. In a 30 mile drive you would have to change the transmit frequency of the phone and the frequency of the radio 3 times...

Your best bet is something like >>THIS<< using the phones Bluetooth.