Calls create completed 'call' tasks. When a Task is completed, you can have the option of assigning an outcome for a task. You can have as many Task Outcomes as you wish, or none at all. All Tasks Types that have been configured to have an Outcome, share the set of defined Outcomes. If a Task Type is configured to have an Outcome, the user must assign an Outcome when completing any Tasks of that type.
To remove the call disposition, Administration->account settings, go to the 'Tasks' tab, edit the task type for Call
Uncheck the associate outcome option.