This attribute, if set to a value of allowed indicates that outgoing
high priority calls (from the DTE to the network) can be made using
this particular Dna. If set to a value of disallowed, then such calls
cannot be made using this Dna - such calls will be cleared by the
local DCE. This attribute is ignored if the corresponding attribute,
outCalls is set to a value of disallowed.
This option can be also included in X.25 signalling, in such case it
will be overruled.