At the beginning of each player's upkeep, that player sacrifices a non-Zombie creature.
At the beginning of the end step, if no creatures are on the battlefield, sacrifice Call to the Grave.
9/22/2011 If a player controls no creatures, or each creature he or she controls is a Zombie, that player does nothing when the first ability resolves.
9/22/2011 If there is at least one creature on the battlefield at the beginning of the end step, the last ability won’t trigger. If it does trigger, but a creature enters the battlefield before it resolves, the ability won’t do anything when it resolves. Call to the Grave’s controller won’t sacrifice it.
9/22/2011 In a Two-Headed Giant game, Call to the Grave will trigger twice during each team’s upkeep, once for each player. Each of the two players will sacrifice a non-Zombie creature when the ability referring to that player resolves.