Under sample/callback, in the directory of Tiger Dial installation there is a filed called index.html.
Here you can find a form for letting your customers to leave their phone number; in this way Tiger Dial can call them when some Agents are available.
It is based on javascript with jquery and css. The related libaries are under the same directory of the index.html file.
You can customize the form according to your needs, for example you can add it on your website.
A basic example of callback, instead, is called callback-basic.html.