This attribute is an extension of the Closed User Group (CUG)
facility, as follows:
This attribute, if set to a value of allowed indicates that incoming
calls (from the network to the DTE) from the open (non-CUG) part
of the network are permitted. It also permits incoming calls from
DTEs that have Outgoing Access capabilities. If set to a value of
disallowed, then such calls cannot be made to this Dna - such calls
will be cleared by the local DCE.
This attribute corresponds to the CCITT 'Closed User Group with
Incoming Access' feature for Dnas in that incoming access is
granted if this attribute is set to a value of allowed.