Turning off the Iphone "radio"

Is there any way to turn OFF the Iphone "radio"? I had a blackberry before and there was a radio button that let you turn off the signal for the phone. I use the Iphone to play Pandora from my stereo and as it is playing - the phone is searching for a signal etc and creating very annoying buzzing and popping coming through the speakers!! Where's the "off" button for the signal??