The Devil's Game (2008)

Struggling artist Min Hee-do is offered three billion won to bet his life to a game against a rich old man, Kang No-sik, who is dying from a terminal illness. The game is for each man to dial a random phone number and guess if the person who answers will be male or female. Hee-do loses the bet, and after a month long brain operation, he wakes up to find that they have swapped bodies.