Q: Phasing (described on pp 1-2 of the Mirage rules) seems to imply that a creature with this characteristic is in play only half the time; i.e., a turn after it is summoned it phases out, and a turn later it phases back in. Is this the case? Does the owner have any control over the phasing ability?

A: The phasing ability just happens. If the creature is in play at the end of the turn it phases out. If its phased out, it phases in. While you might control the phasing creature, you have no control over when it phases.