This is correct. I've flown in HAWC 1 on two occassions. Flight Officers monitor various channels during their shift just like we do when scanning. They set one channel (usually A1 Districts 1/2) as a priority channel.
When necessary, they are dispatched to a call by the dispatcher. this can be in one of two forms. 1. The dispatcher will contact them on whatever primary channel they are on or 2. more often the dispatcher will "go all channels" and will hail them (i.e. HAWC 1 to A3 ... HAWC 1 to A3).
Hope that helps.
Sean - very new to this website but so happy to have found it!