You have to get Chang to call off some of this homework.
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Their meanings overlap a great deal, but there are some differences in where you use it.
To call off something naturally comes from cancelling something by shouting out an order, but has spread to mean almost any form of cancelling.
In some expressions it could be confusing to use call off instead of cancel, for example when cancelling a subscription.
Similarly some uses of call off can't just be replaced by cancel, for example in the expression call off the dogs. To use cancel instead, you would have to specify that it's the action of the dogs that should be cancelled, not the dogs themselves.
They have some special usages: