When you create a new Callflow, you pass a list of numbers. I think you are looking for this API doc here https://docs.2600hz.com/supported/applications/crossbar/doc/callflows/#create-a-new-callflow
you also can update an existent CallFlow and add more numbers to it https://docs.2600hz.com/supported/applications/crossbar/doc/callflows/#change-a-callflow-object