The calling is a form of magic that a Warlock may have in order to manipulate the world around him. The calling is different from the Gift in that a person is born with the gift but the calling is when you want to become a wizard and are taught to use magic, usually by a wizard with the gift. The calling can be for either Additive Magic or Subtractive Magic. The subtractive magic can be taught by a wizard that has the subtractive gift or calling or by the The Keeper.
How one could manipulate Han without the gift Edit
The Calling was explained in Naked Empire to start out as a spark of the gift that is in every person (except the Pristinely ungifted) that has been multiplied by a wizard. This spark is required to be able to interact with additive magic. The way that a person who wants to be a wizard could use magic is to have their spark added onto by a wizard's magic. This will give the person with the calling a weak form of the gift that basically only allows you manipulate the world. The Subtractive calling is taught the same way but has only been done by The Keeper and possibly Sisters of the Dark.