Normally, they do respond best to higher pitch sounds as mentioned above. Such as bird sounds, or young cottontail or rodent if using an e-caller. The one key I would state here is to "back off" on the volume with reds though. Reds don't care for the volume as much as a gray will tolerate. If using mouth calls, get yourself a Johnny Stewart PC-3 or an Ed Sceery AP-6, both are bite type calls and you can get out on the end of the bite plates and really get some rodent or higher pitched sounds, just use real short and rapid breathes. With all this being said, you never can second guess a fox though. I have had reds come in when playing the FoxPro Aggressive Jackrabbit while calling coyotes with pretty loud volume and this is a very raspy and deeper sound, far from what was described above.
As far as best times, the guy above that said the best time to call is anytime is correct. "Get out there," you won't get er done sitting on the couch. But I have found the best times are twlight, last and first light.
"THE BEST ADVICE YOU WILL BE GIVEN, IS TO LEARN TO LIPSQUEAK." I have had foxes show me, through their body language, that they have had enough and was leaving the area with wreckless abandon. As I started to lipsqueak they would slam on the brakes and turn towards me and immediately start stalking the sound, usually coming pretty quickly.
Thanks for the endorsement JAMaidens, I appreciate that.