The username can be found in company account, in the great plans section in the field DCS Trunk Name:
Additionally, you can find the original credentials (Hostname/username/password) that were created for the customer by looking the activation case, please note that the password may no longer be valid if the customer has requested a password change.
If the password has changed (and the customer either does not remember it). you will need to contact the DCS team so they can reissue a new password.