Hi
If you don't have a lamp, calling them in the early evening before it gets dark can produce good results, especially if the ground is not hunted. Though around now there are plenty of "green" youngsters about that will on occasion run right up to your feet. If you are trying to stalk or get the drop on foxes on foot, you need to start looking into to the mid and far distance, rather than close up like rabbits. The wind is the most important factor to consider, foxes when coming to the call will often circle approach you from downwind to check you out. Baiting an area with roadkill can be quite effective, as is snow, frost and hard weather, casualty livestock. In the mating season they get quite cranky, abandon natural caution and are easier to call and more vocal.
Cheers