Battle Cry is a triggered ability that triggers when the creature with Battle Cry attacks. When this triggered ability resolves, all attacking creatures apart from the creature with the Battle Cry ability will get +1/+0. This effect lasts until the end of turn and does not end when the combat phase does. If an additional combat phase is generated and the creature with Battle Cry does attack again, this will cause attacking creatures to get an additional +1/+0 until the end of the turn.

One final thing to note is that in two-headed giant games, the effect from a Battle Cry ability will affect attacking creatures controlled by a teammate, as all attacking creatures are declared at the same time in a shared turn.