Retrieves a sub-account request made through POST /sub-accounts. status reports where it stands (Accepted, Active, Failed, Pending, Rejected): Pending until onboarding has consumed it, Accepted once the sub-account exists and its virtual IBAN is being reviewed, Active once the IBAN is allocated and the sub-account approved, Rejected when refused by validation or review, Failed when the IBAN could not be allocated. subAccountId and iban are set as they become known. A request that does not exist or was made for another client returns 404.