All calls have their place on nights when nothing else works,but,in my experience nothing works every time,I do have a U-Caller and at times it does work excellently calling foxes from seemingly miles away,but as I say it doesn't all the time,and I think if you can use your hand to call and are able to change the way you call at different times then that will be more effective,also if you lamp with a partner then mix it up a bit,and get them to call 50% of the time.
Martin