The advisory routes are
Class F airspace which anyone can fly in and follow either Visual or Instrument
Rules (VFR or IFR). If you wish to use the advisory route structure and the
various ATC services, i.e. you want to participate in the advisory route system, you have to file a flight plan.
As participating traffic
ATC will give you separation from other participating traffic but the airspace
is still available for use by non-participating traffic.
You can comply with
Instrument Flight Rules (fly at the appropriate level for your magnetic track
etc) without talking on the radio.