Calling REST API while viewing completed task/case

I have below requirement to be precise .


  1. Based on user input in one text box rest called will be fired and other two text boxes will be filled in.
  2. Values in these two text boxes will not be stored in database since data is sensitive.

Related task is completed, now while viewing the form attribute of that same task can rest interface be called to get the values again.

Or after completion of task external interfaces can not be called?