Better pelts in the deep winter. Been my experience Fox are few when Coyotes are around. My best results for Fox was when I'd spot the dens and catch them coming or going. Both Fox and Yotes tend to make the rounds of their territories. They find Rodents in the same places and visit those places often.
One almost surefire place to set up is on a freshly mowed hayfield. The mower chops up a lot of Rodents and makes easy pickings for many predators. Another good spot is by Hay rolls left out in the field, they are also often full of rodents and Fox will use them as dens.
I've rarely used calls, mostly I spot their movements and ambush. A couple of tricks, they tend to avoid the wet grass early mornings. I set up on the farm roads, they hunt the verge and ditches next to pastures and crops. I sometimes set up on a hilltop with good glass and check out the countryside in a broad sort of way. Print out a google earth map of the area and mark the spots where you spot Fox or Yotes. Then plan an ambush, Or spot a well used trail which is easiest after a rain. Like I said they aren't gypsies, they tend to makes the same rounds in their territory as long as there is food.
My way of hunting isn't sure-fire, but my success rate is around 40%, for days spent waiting in ambush or scouting.
One technique I've used with success is a ton of manure piled over old chicken bones, offal, leftover kitchen waste, old cooking grease or whatever protein source you can find easy or cheap, In the deep winter the manure pile stays warm and throws up a predator yummy scent cone. Even the Rodents will move into the pile and make it doubly attractive to Predators.