Um, actually the range on a RKE fob with good batteries is about 300 feet. Try it across town. Does not work.
For that car thing to work, the fob must be soun based (OLD tech) and the phones must be high end. If the fob is a modern RKE radio frequency fob, the following would have to happen: the microphone in the sending cell would have to act as an antenna, the amplifier connected to tha mic would have to work on an 800,000,000 signal when it is designed for 100 to 20,000 Hz, the phone would have to encode the 800,000,000 signal on a 300,000,000 Hz carrier (impossible), the entire cell/PCS network between the phones would have to carry the 800,000,000 signal intact, the receiving phone would have to decode it properly, and the 20,000 Hz max speaker on the receiving cell would have to work as an 800,000,000 transmitting antenna. Not gonna happen...