A car door can be unlocked using a cell phone if the vehicle comes equipped with a security system that allows it. OnStar, a fully integrated vehicle security and communication system, allows drivers to unlock their cars using a mobile app.
Various articles claim a car can be unlocked through a cell phone by calling a second party with a spare set of keys and having the person press unlock near the phone's microphone. However, Snopes reports that this does not work. Remote entry devices use radio frequency signals to transmit the unlock command to the vehicle, and cell phones cannot relay these signals.