How do I stop incoming calls from interrupting my game?

Recently I was playing a game and was in the middle of a tournament mode with another player and I was about to win the tournament and I got an incoming call and it disconnected me from the game and ruined everything. Is there a way to turn off incoming calls while I play the game? Airplane mode won't help me because the game requires an Internet connection, so I need airplane mode off so I can play the game.