> You also don't need to concentrate much to listen to the radio, unlike talking to someone on a cell phone.
that depends on what you are listening to.
listening to a talk radio host that has you yelling back at the radio is almost certainly as distracting as listening to a phone call :-)
listening to background music is not going to be that distracting, although listening to music that you like well enough to start singing along and moving to the music probably is far more distracting.
listening to books on tape could also range wildly, most of the time it's not going to eat that much of your attention, but when you get to the cliffhanger section of a thriller, It can eat a lot of your attention.
The effect of all of these distractions vary greatly from person to person.