If call back is not configured the option won't show and it would make the message ridiculous for off time.

Specs says:

As soon as the the customer opens the gadget, it should display the following message 
"There are no agents available at this moment. Please come back later or select an option from below. Thanks".

The message should precede the following two options:

1. Callback [The option should be disabled if callback module is not available]
   • Choosing this option should open the callback UI.

In case of call back is not configured there won't be any call back option and the message says see option below... please rephrase the message and update the story.