One day my friend left his car key inside the car. So he phoned his wife to bring the spare key. She accidently push the unlock button while talking on the handphone to her husband and the car is unlocked. I tried myself but nothing happened. Do you think it is possible to do that?

yes its possible already. But I dont think that sort of feature is intergrated into cars and phones yet, but within a few years most of your stuff will be controlled by smart phones.

But wouldnt the Car need an IP address or something to do that.

Some cars have the system (forget the name, I think it's in Hummers and other larger cars) where they could push the button and ask an assistant for directions or their own location, you could call them and they can unlock your doors if you locked your keys in, after you prove your identity. This would be one way of unlocking your car using a handphone, probably not the same way you were thinking though.

OnStar

BMW Have a smart card that emits a signal to the locking system when your within two feet it opens the door and locks when you move away you just need to keep it in your inside pocket and forget about it. Where I'm from a brick does the same job.

OnStar does it for you to. Just call them up.