Lg800g Need Hooked To Car Radio Auxilary To Be Able To Talk And Hear On Speakers

want to hook to my auxilary on my ford truck radio and be able to talk and hear on truck radio speakers. what do i need to be able to go hands free like this ? i purchase a cable from radio shack and hooked ti to my phone and to aux. i could hear,but could not talk to the caller. my son's both phones(blackberry/droid) will both work tpo talk with and hear using this same cable. wondering what is wrong my tracfone will not do this also ?