This attribute, if set to yes indicates that this Cug is the preferential
Cug, in which case it will be used during the call establishment
phase if the DTE has not explicitly specified a Cug index in the call
request packet. If set to no, then this Cug is not the preferential
Cug.
Only one of the Cugs associated with a particular Dna can be the
preferential Cug - only one Cug can have this attribute set to yes.