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When the call is initiated a guid will be created; the data will be stored by calling setCallbackData(guid,data) and the guid will be set as channel variable (nj-agi-callback-id) on the newly initiated call. In Routes this may be accessed by calling getCallbackData() as Routes has it's own method that knows about retrieving the guid from the channel variable.
While Callback Recieved expects that data will be of the form Map<String,String>, you may pass other data types. Callback Recieved will however not retrieve the data for you if you pass another type, you will need to make a call to ActionScriptUtils.getCallbackData(guid) to get the data.
Object getCallbackData(String guid)
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