This attribute, if set to a value of allowed indicates that incoming
calls (from the network to the DTE) can be made to this DNA. If set
to a value of disallowed, then incoming calls cannot be made to this
DNA - such calls will be cleared by the local DCE. This attribute
corresponds to the CCITT 'Incoming Calls Barred' feature for
DNAs in that incoming calls are barred if this attribute is set to a
value of disallowed.
Either outCalls, or incCalls (or both) must be set to a value of
allowed for this DNA to be useable.