Since the phone is already dead, calling the phone would be unnecessary. I would advise you instead to call the taxi driver in case you happen to know his number. Ask him whether he has found any phone in the car. If it turns out that no one is tracing the phone and you are certain that you have lost it completely, notify your service provider so the phone's line can be canceled. It would be good to install your phone with tracker software though this should be done before you lose the phone. Read more from http://www.howtodothings.com/electronics/how-to-find-a-lost-cell-phone.