This attribute, if set to a value of allowed indicates that outgoing
calls (from the DTE to the network) can be made using this
particular DNA. If set to a value of disallowed, then outgoing calls
cannot be made using this DNA - such calls will be cleared by the
local DCE. This attribute corresponds to the CCITT 'Outgoing Calls
Barred' feature for DNAs in that outgoing 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.