ErrorApplication error64021: Pay: Invalid OperationPositive FeedbackNegative FeedbackTwilio Voice DocumentationVoice Descriptiondescription page anchorPositive FeedbackNegative FeedbackA 64021 Error is an indication invalid operation was provided. Possible causespossible-causes page anchorPositive FeedbackNegative Feedback Start operation on already started Pay session Possible solutionspossible-solutions page anchorPositive FeedbackNegative Feedback Make sure to use a valid operation