Call to the Grave

• WWizards of the Coast2011-09-22
  Official Ruling
  If a player controls no creatures, or each creature they control is a Zombie, that player does nothing when the first ability resolves.
• WWizards of the Coast2011-09-22
  Official Ruling
  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.
• WWizards of the Coast2011-09-22
  Official Ruling
  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 before it resolves, the ability won't do anything when it resolves. Call to the Grave's controller won't sacrifice it.

Rulings via Scryfall