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.