Call back is two words when used as a verb.
Example: I need to call back two of the top candidates for the position.
Callback is one word when used as a noun or adjective.
Example: After the audition, Chaital hoped for a callback.
Call out is two words when used as a verb.
Example: You should call out the processes in the diagram.
Callout is one word when used as a noun or adjective.
Example: The illustration had a callout created as a text box.
Callouts are text labels in an illustration that point out specific items that should be noticed by the reader. (See Figure 2.1.) Consider the following tips for using callouts:
Callouts should have the first letter capitalized.
Do not capitalize ...