OnStar is correct, on one condition
This would work -- but only in AUDIO rather than RF.
Much like an encrypted sequence of touchtones (an encrypted audio key, so to speak), to transmit an unlock code to a car... If the car has a microphone that listened to an audio "key", this could potentially be possible, even if the code was changed. Yes, even if the code changes every time - since the authentication would go two ways through the cellphone (headpiece and earpiece), allowing changeable codes to work. Yes, it could technhically be made to work. Yes, it would still be completely safe from recordings (i.e. taped recordings won't work) because the audio key is changed every use.
I would have to call BS on RF though...
But, yes, I say it is possible on audio frequencies transmittable through cellphones... Provided the car has a microphone instead of an antenna. It's definitely technically feasible and doable, assuming the codecs in the cellphones don't scramble up the "audio key" too much.
The question is, are those "working" cars really using audio instead of RF?
Remember, not all key fobs use RF!!! Some use infrared, some use sounds.
The ones that uses sounds, are the ones that will "work" over a cellphone. These are NOT RF.
The early key fobs used the same code.... Such as infrared, which was learnable by a programmable remote control. So that a thief could just use a remote control to unlock a car. A tape recorder would be an equivalent risk. However, if the key changes every use, then...
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