Set your ecaller up in a relatively open clearing, but near enough to an edge habitat cover that the terrain will allow a predator to have some confidence of getting to within 20 yards of it without being busted himself. This will work a lot better on bobcats and grey fox than putting an e-caller out in the middle of a large open field.
Remember, too, that the parallax error will be greater the closer a predator is to the caller, meaning you're often less likely to get busted 50 feet from the caller in close quarters (or hand-calling, for that matter) than you might be 50 yards from a e-caller set way out in the open-- provided of course, that there is parallax, ie that you're off to the side and not in a direct line with the e-caller in the direction the animal approaches.
Both of which get to the issue of stand selection--IMO the most critical factor in all of predator calling, ecallers or handcalls alike.
LionHo