You can pretty much plug any audio signal into that jack.

If you were clever enough, you could take the typical 3.5mm connection of a cell phone, split the output from the input (speaker from the Microphone line) and connect it to the 1/4" input jack such that your caller would be heard through the radio. You could install a Microphone in the car and have it as a handsfree system. The only risk is overmodulation/distortion in the event the signal was too loud; but the volume on the phone could handle that part...