This attribute, if set to normal indicates that the default priority for
outgoing calls (from the DTE to the network) using this particular
Dna is normal priority - if the priority is not specified by the DTE.
If this attribute is set to high then the default priority for outgoing
calls using this particular Dna is high priority.
This option can be also included in X.25 signalling, in such case it
will be overruled.