DBU on the DSU IV ESP is triggered when at least one of the DBU Criteria is ENABLED and that condition is met. The DBU Criteria that is most commonly used consists of OOS/OOF errors, LOS, and ALLONES. The DBU Criteria for Sealing Current should normally be DISABLED unless the provider has verified it is present all the time.
Other settings like Automatic DBU, Number to Dial, Origin/Answer must be specified in order for DBU to function. For more details please reference the DSU IV ESP User Manual.