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
DTE's 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 ITU-T '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.