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 ITU-T 'Incoming Calls Barred' feature for Dna's
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.