This read-write OCTET STRING holds the actual E.164 ISDN address (phone number) for this entry, stored as a plain digit string, for example the number 15086922600 is stored literally as the string '15086922600', and the description notes that setting this to a null string removes the entry entirely. This is the dialable number the switch associates with a given logical ISDN port for call routing purposes. An engineer decommissioning a number would SET isdnAddr to an empty string specifically to delete the binding, rather than needing a separate delete operation.