Requests a call back to a given number by an operator in a given call center. The call back is added to the queue for the call center like a regular call, and a call is initiated when the next operator becomes available. This API respects all existing call center settings, e.g. business / holiday hours and queue settings. This API currently does not allow international call backs. Duplicate call backs for a given number and call center are not allowed. Specific error messages will be provided in case of failure. Added on Dec 9, 2019 for API v2.