This attribute, if set to normal indicates that the default priority for
outgoing calls (from the DTE to the network) for 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 also be included in X.25 signalling, in which case it
will be overruled.