I was a skeptic until I had a conversation with someone who shoots over 800 a year as a pro pest controller, I use a Icotech nightstalker and I have had brilliant results with it over the last 2 years, common mistake is being lazy and putting it out 10 yards because you can't be bothered to walk 80 yards, Playing too loud, I start on 5 let it play for 5 minutes, normally you will have a result by then, if not go up a level for 1 cycle or loop, Try and pick the right call for the conditions, if you have stubble or grass, I have found that a mouse type squeak works best played at a mid volume and reduce as it's coming in, From now until end of February is a great time to use the Vixen in heat call< if no results within 30 mins go to another stand unless you know there's one in your area.
Over the last 2 years I have shot over 200 Foxes all to the call, so It does work, but dont' play too loud for too long.