This attribute specifies the index of the Closed User Group with
Outgoing Access facility to signal. A value of 'DEF' indicates the
use of the value from the cugoaIndex attribute of the X25Dte
component. A value of null (empty string) indicates no CUGOA
facility is to be signalled. If a cugoaIndex is provisioned to be
signalled, the cugIndex attribute must be null.