A callout shares many of its properties with frames (which is not surprising, given that a callout is a frame with a line). See Modifying frames for more details.
To modify the properties that only apply to a callout, follow these steps:
Click in a callout to make it active.
Right-click and select the Frames command from the shortcut menu (or select the Frames and Callouts command from the Format menu).
Select the Callout tab in the dialog and set any of the following:
Attachment - the callout line can join the frame at the nearest Corner or at the nearest Middle of a side. Select None to convert the callout into a plain frame.
Arrowhead - modify the appearance of the arrowhead. The default is to display no arrowhead, that is, the callout will be a plain line. Select an arrowhead Style to turn the line into an arrow and determine the arrow’s direction. The width and the length of the arrowhead itself can be set using the Width and Length drop-downs respectively.
Line - set the thickness and style of the line using the Line Style drop-down and the color of the line, which includes the color of the arrowhead, with the Color drop-down.
The results are displayed in the Preview box. Click on OK when you are ready.