This attribute, if set to allowed indicates that incoming calls (from
the network to the DTE) be made using this particular Cug. If set to
disallowed, then incoming calls cannot be made using this Cug -
such calls will be cleared by the local DCE.
This attribute corresponds to the CCITT 'Incoming Calls Barred'
feature for Cugs in that incoming calls are barred if this attribute is
set to a value of disallowed.