Call Pet is used to summon a hunters pet that has been dismissed or has run out of range and automatically disappeared.
If you use Dismiss Pet when in battle, casting Call Pet will bring back the pet with the same Health as when it was dismissed.
If the pet auto despawns because of range, casting Call Pet will bring back the pet with Full Health.
Tips and tactics
Call Pet is very useful in situations where you have had to run away from a mob and your pet is almost dead. Send the pet after something a fair distance away, run in the opposite direction and when out of range the pet will despawn, when you cast Call Pet, the pet will reappear with full health.