There is a built-in stencil called Callouts in the Visio Extras category, but the Callouts introduced as part of Structured Diagram API are something different. These are a special type of Shape that have some programmatic control. Any shape can have many Callouts associated with it. The names of Callout Masters in the built-in stencil can be read by moving the mouse over them in the Insert | Diagram Parts | Callout drop-down gallery, as in the following screenshot:
The following code,
AddCallouts(), gets a reference to the Thought
Bubble callout from the built-in stencil. It then loops through all the shapes in the active ...