All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
The Sharia ruling concerning competitions rests on the origin of each; therefore, if the organizer collects money from the participants, or they are liable to win or lose, then it is considered a form of gambling because it involves eating up people`s money unjustly. Allah, The Almighty, Said in this regards {What means}: " O ye who believe! Intoxicants and gambling, (dedication of) stones, and (divination by) arrows, are an abomination,- of Satan’s handwork: eschew such (abomination), that ye may prosper." {Al-Ma`idah/90}.
However, if the organizer doesn`t collect money from the participants, or someone else had donated that prize, then there is no harm in benefitting from it so long as both the origin of the competition and the origin of the prize are lawful.
Accordingly, if the telecommunications company has deducted more than the price of the SMS from the participant`s credit, then the competition is considered a form of gambling, which is unlawful, and so benefitting from the prize {car} is prohibited because the only channel for ill-gotten money is charity. Nevertheless, if no extra amount was deducted, then it is lawful for the winner to take that car. And Allah knows best.