Foxes can make all manner of weird sounds - the scientists reckon they have at least 40 different vocalisations. Having said that, I've had the odd individual with what appears to be a unique call - I well remember one that made a noise like the siren on a fire engine! If the fox is not interested in your calls - and I've had to deal with countless ones like it, then you're probably wasting your time as someone else may have made it call-shy or it might just have a different agenda. I'd suggest that instead you simply put some bait down and get it that way.